ext_data/weapons.dat is only parsed for the SP game. It controls things like firing rate, etc.
We can't change the code for SP, because we don't have it.
For MP, we can't add in more weapons without taking out others. (Unless your mod has a serverside and clientside, and you manage to hack up the engine and tweak the networking so it sends playerState_t::stats[STAT_WEAPONS] as a full 32-bit integer, rather than only sending 19 bits. (Hence 19 WP_ slots)
You can't just 'open up' someone's .dll and expect to see their source code. That's just not how code is compiled and dispatched.
You'd have to learn coding yourself and create your own mod from scratch - Which ain't no weekend retreat :/
If you do choose to learn coding, there's a few tutorials on obtaining and setting up the SDK, and there's still a bunch of us coders around to help - but the best way to learn is by doing.