welcome to the forums.
modifying is not enough, you need to inject some self written code. basically you need to look at some of the weapon class functions (you should find these with the pdb) and create a new class inheriting from it. when this is done you need to create a prototype class and register it within armada.
creating the inherince class is the biggest part of work here. i did this entirely in delphi. i don't know if you can do this easier in c++ as i did in pascal. i had to recreate everything from ground up that means even virtual function table and object descriptors as obviously delphi uses its own data structures for classes. as armada 2 was programmed in ms c++ 6.0 it may be easier to create a new class with c++.

you need to write a hooking dll and loader, that attaches the dll to armada (google for hooking). it's alot easier to write a new weaponclass from scratch as modifying the existing one. you need to call a lot of armadas existing functions for that. i don't know how that works in c++ but you could always do some assembler calls.