I find the easiest way to edit the game is using the DME program, if you go to McFlys site, he has a resource centre and tutorial on the DME, there is no manual and Im afraid its one of these "get your sleeves rolled up" things and just experiment.
All weapons are configued in this program including a AA12, but it will only be able to use one of the other missiles envelopes heat seeker, IR, homing envelope etc.
J