To be honest, it doesn't look like you will ever be able to add weapons to the game unless you actually edit the game yourself which cant really be done since the source code is not available, to my understanding
and they have told us that they wont be doing it either, so you'll have to live with the existing weapon set. you can modify the weapon graphics and statistics though, but the statistics (fire interval, ammo, etc) will only be avaliable for use offline or if you host it on your own server.