The best way I can think of is to create the truck in a 3D modeling package, seperate the truck and the turret into different 3d files, and add bones in truck for reference where you want the turret attached.
Then in blitz3D, just find the bone(entity) and fix the turrets location to this point..
Then in blitz3D, just find the bone(entity) and fix the turrets location to this point..