find the classname of the addon vehicle by creating a test mission with it in and then scanning the mission.sqm for its entry
and then when you have it, createvehicle in the same way you would for any vehicle
_vclpos = getpos (Gamelogic called whatever)
_vcl = "Vehicleclassname" createVehicle _vclpos
removeallweapons _vcl
_vcl addweapon "WEAPONCLASSNAME"