Still no reply from that other fellow...
After modding the latest development version of ufo:ai (started from scratch because even though I found what was changed in scripting syntax and made appropriate mods, the game still exited to desktop with no error!) same problems remain, cannot create new enemies (only modify alien models) and weapons system hasn't change at all. It's still possible to mod it however but weapons cannot exchange ammo even though they use the same kind. I'll try to create an alpha version using existing models from the game.
@ inquisiteur2
If you have any more weapons models please send to my email (I 've send it with pm)