You need hmt and the plugins.
Choose your map, go to Vehicles, and select the turret. Change type from turret to alien fighter or human plane. Give it all the values of the banshee up to seat. Save and test. if it moves most likely you did it right. you might have to give it banshee physics but I don't think so.
I know its possible on halo 2 because in halo 2 the vehicle turrets are under the vehic tag as well so you probley could do it on halo 1 ill give it a try tommorow cuz thats when i am playing halo pc again