Welcome to FritzBot.
FritzBot is not open source so the source code is unavailable to all (except Maleficus).
So there is no way to add new weapons properly.
However if all you want is the sound and look of a shotgun the problems on doing that are discussed in some detail in
another thread. But weapon damage and ammo is server coded so you can't change those.
FritzBot is really targeted for players who like the original ET. If you want extra weapons I suggest you look at Bobot/Omnibot etc.