PXSchemeEd has a weapon editor, which is accessible via Edit button in Weapon Tables zone. In there you can set count of any weapon by using Count field. To set infinite count enter -1 into that field. To add a custom library based weapon, add appropriate lib, go to the weapon editor, and use "Use from library" zone. You can also set count for those weapons using the same method I believe.
Don't forget to check Count btw, if you don't do that it will use standart scheme's count and will be editable ingame (it will look like one of the default weapons though).
http://imgur.com/a/cyv3KIt will replace the chosen default weapon though. It's possible to not replace a def weapon by using a second weapon table, but I personally don't like switching tables ingame (which is done via '+' or '-' button). I hardly even remember how to do that honestly. I might be wrong on the button. Maybe it was F12? Or a numpad button?
Also:
http://px.worms2d.info/px080wiki/index. ... r_Tutorial