The problem in modification of this game is that Fonline team took closed source approach. Yes - they rely on old fallout files, which will be closed for another 20-50 years, but they, IMO, made a mistake by releasing closed source of their wrapper-engine.
So you have 2 choices actually. Get the undat - extract critter.dat and master.dat and try to understand the closed-source file structure and scripting. Another choice is to hack the client itself.
I believe guys will provide some kind of interface for modifications in the future, but I dont expect it to be soon.