I know you can use Cubik's item editor to enhance the possibility of knocking out a critter. While enhanced, I don't think it is 100 percent tohugh. That is just by modifying the weapon itself (proto file).
However, I am fairly certain that a critter can be scripted to be knocked out if a specific item were used on it. Not a problem if you only want the item to have a very limited use in the game. If you wanted it to work on all critters, it would mean modifying many, many scripts.