This tutorial shows how to modify a csv file, but unfortunately I still dont know how to apply it to the game. If someone finds a soltuion, please feel free to post it!
It was written for the PS3 version of the game, but it might be also usable for PC users.
The xml-spelling-rules for writing articles here are really annoying, so I wrote my theoretical tutorial on modding weapons for RE Operation Raccoon City in 2 files, and added my modified weapons.csv as a download.