Sorry I just now saw this post. I've done it a few times for people carrying Spydercos or some other FRN/Zytel type knife and even some Ti and aluminum model knives that were pinned. It varied how folks wanted it. Some wanted just to be able to take th blade out when they wanted and/or alleviate the side to side play that is common in these nylon reinforced handled knives. Over time they seem to just get some wiggle going and it bothers some. I've never seen it stop one from really using the knife and I know a lot of good ole boys with a slippie (slip joint) in their pocket they've worked pretty good for a number of years and keep on workin' even with some side to side play.
If thats all it is you can use one of the old style Gillette razor blades and shove it down in between the blade and the handle close or until it touches the pin. Leave that in there and then peen the head of the pin and you can then extract that razor blade out of there once its tightened down for you some. That seems to give enough gap to allow for some smooth action so you don't overppeen and get carried away by going too far for tightness. You don't want the pivot so tight that the pivot tension can over power the back spring because when and if you do this the tip of the blade can expose itself in carry and the spring can't make it come back down. Could be a bad deal there if you make that tension too tight, and that can happen for swapping a pivot barrel in there also.
You may end up after this peening trick with a tiny bit of side to side play but it should be better. Now if you just want to install a barrel in there what I did was order some 1/8" stainless pivot barrels from Tracy at USA knifemaker supply and a carbide 1/8" diameter two flute drill from MSC. Don't get the twist drill carbide. They do cut okay but if you hiccup using it they shatter and all they ever done for me is make choice words usually associated with sailors come out of my mouth. So get the two flute and if they don't have that you can get by with a spade carbide as a second choice. I think the spade may be a little less painful to the wallet so if thats an issue get that and it should do the job just fine.
Okay so now you have your folder in front of you. Lets pretend for arguments sake its a FRN Salt 1 by Spyderco. I use my dremmel to remove the heads of the pins. I only take off the smaller side head. If you look at the pivot pin in there you will note one side has a bigger head than the other. Take it and turn it so its got the smaller head staring at ya. With a small diameter thin kerf cut off disc like Keystone makes or like you can buy at a Sears or Lowes in the dremmel section you can take that head off pretty easily. Stick a 1/4" or so diameter (outside diameter) cut off disc in a mandrel. Variable speed dremmels work best for this cause you ain't going to the races here you are just taking the head off a pin in little baby steps at a time nice and neat. You want a smaller outside diameter cut off so you don't chew up your FRN folder body needlessly and since there is a washer under that pin head that is steel you can chew that fine and get by cause when you are done that washer can be flipped over so the bad side is down and look good as new on the outside once the pin is out.
Whew! Okay where was I? Oh yeah. I sometimes use a well used cut off that used to be a bigger diameter. If all you can find is a bigger 1/2" ones for outside diameter or something even bigger around you may just have to cut off some nails and reduce the size of it some to save your FRN from gettin' all chewed up when you remove the head of the screw. No big deal and it won't take long trust me. The secret to using those thin non reinforced cut offs is to let the disc do the work. Don't be pushing it now or you'll shatter the thing and have to start over with another disc. and besides that you don't want it running away and getting a line marked on your folder. So do it lightly and just let the thing do all the work while you hold it steady. Once its the size you like just use it to cut that head off nice and neat and it will zip that head off there right quick just letting it do the work for ya and you have pretty good control. Trust me I've tried the drill bit taking the head off routine and I've tried other things too but this way works for me the best and is the quickest easiest way I've found to get a head off a pin without having to fuss with the pin spinning in the pivot just as fast as the drill bit.
Once that head is off there you need to punch that pin out or use an arbor press and just push it out. If you don't have an arbor press use an upside down drill bit in your drill press and use that. Should come out of there okay with a little pressure and it can help if you can depress the lock. Do that without gettin' cut preferably!! Speaking of that if you are smart you'll just tape the blade off first thing to keep that edge from biting you or gettin' dinged all up while you work.
Now that your pin is out and your blade is free you can either do the same with the lock bar also called a rocker arm or you can just stick to the blade. Now I'm gonna warn ya the rocker arm can be tricky but if you go there prepare to be a bit concerned if you have no way to hold it steady to drill a new hole in it cause that is what its going to take to put these barrels in there. What you want to do now is crank the drill press up to high speed. Tape off that cutting edge but good and with a gloved hand and some help from a clamp on a 1x6 block of wood make sure that blade and 1x6 are secured down to your table on the press. Letting the drill bit do the work you want to zip a 1/8" hole in that pivot now. That drill bit will do it just be extra careful when its ready to perf through to the other side as thats when these things tend to break. Just let it do the job and take it down there nice and easy a bit, let up, blow the shavings out, take it down again a little and let up, like that. I've found this keeps the heat down, and it is less likely to shatter your bit. So what I"m sayin is don't just try to force the thing through all at once like you would using it on something softer. Remember you're drilling hardened steel now.
Alright, you have your hole in the blade now. Now you want to position that bade in the FRN handle to gap it correctly but you don't want to shove it in there too far so its in the path of your drill bit. You're just using it to gap the two sides of your folder body for spacing. Now zip the 1/8" bit through that FRN handle nice and neat but carefully so you don't go off on it and drill a new hole. You just want to enlarge the existing ones and if you like you can slow the press back down now to a lesser speed. Not much going to matter at this point drilling that. Due to the type drill bit you have you won't have to worry about it pulling the piece up toward the chuck like a twist drill either which is nice. So just make your holes, and then while depressing the lock bar you insert your blade. Test fit that 1/8" barrel in the pivot you just drilled and make sure it goes in there first. If not you may have to do some reaming.
The barrel may be long. So once in you'll have to mark it for excess and using some vise grips you can slice that end off just using a thin kerf cut off disc which you should happen to have at this point. Once thats cut to length you can trip the burr of it if any, round it some and flatten with a bench sander. Again don't just shove it in there. Hold tight or use the vise grips and just make it neat. Now you can shove that new barrel in place. Test the washer to see if it needs the hole enlarged. You should luck out here as most of them are oversized holes anyways but it may need trimming to fit your barrel. You can hold that with vise grips and zip it nice and easy too if need be. Then place that washer back in there good looking untouched side so its facing up. If you have a short screw insert it and if not you may have to cut that off some to make it fit too. A small sheet of brass and a hole the threaded part of the screw fits through will suffice here and using a gloved thumb just hold that screw in place, and use the cut off disc once again to zip it shorter. Take yer time though as its going to take a few ticks of the second hand to get it off. Once short though you should be good to go and have ya a nice custom fit pivot barrel in place.
Anyway, its not rocket science but thats how an old fart in the backyard knife shop does it anyways. Its only worked about 40 times or so for folks. Don't need a lot of tools to do it but if you got some patience and a few handy basic tools you can do it yourself just fine.
Should you opt to do the rocker arm hole you really take a risk there. Some smaller knives have rocker arms that pretty dang small. You may find that there is very little room to bump up the hole already in there to a 1/8" size. If thats the case it can be better to just forget it. So its an enter at your own risk kind of thing. Personally I'd leave the lock bar alone unless its a bigger model pinned folder.