A simple solution is to add some commands in the INIT of the object. Assuming your ammo crates is called "MunMag":
MunMag AddWeaponCargo ["M16", 2]
would add two M16. Look also at the following commands:
- AddMagazineCargo
- ClearWeaponCargo
It works fine, but I experienced problem when adding Satchel charges. Maybe someone knows about that ?