It's not easy. You have to unpack the .pbo campaign file and edit each mission individually. --- www.randominsanityonline.com - Please sign up, or FireCrotch will beat me. Call me Shafty. Disclaimer: I am an asshat.
Probably the easiest way to use new weapons is to replace existing ones with the new ones - but that's not quite the same as "adding" weapons. Unfortunately it can be very complicated and there's very little helpful information - the mod community is very odd IMO in that you can find detailed tutorials for unpacking .pbo's and how to make models with Oxygen (I think that's it) but nowhere does anyone explain WTF a .pbo is, how the game uses them, what and why you want to do things to them, what a model is, how the game uses them, what and why you want to do things to them etc. etc. - you're supposed to be freaking telepathic or something!!
Unfortunately all I've done in that area was to replace the original M1 Tank model with a better one - which I'd suspect is much the same process - but that was about 2 years ago and altho it took me AGES and HUGE frustration due to lack of concise info on how the game actually works and uses all the various types of files etc. right now I can't clearly recall how I did it all.
I THINK it boiled down to putting the new item's .pbo into the addons folder then in the config.cpp linking to that .pbo wherever the old item is defined. I did that in the ECP mod - which uses an extracted config.bin - ie. config.cpp which can be directly accessed and edited.
To edit a normal config.bin you'd need to extract it and then repack it and off the top of my head I can't even remember which utility does that as once again, with sweet f-all guidance I had to try several different programs with no idea what they actually did or how until something worked. That was when I was trying to edit the std game's config to add tracer.
All I can suggest is search and try to put the puzzle together.