I'd think you must do this with FireWeaponUpdate logic that fires a permant weapon with very small weapon range and radius (against enemies) in conjunction with the hijacker logic as already said.
@Soviet_Deso
btw: is the drone still alive and available for a new attack after it has destroyed a unit like in RA?
Yes, but is there a way to keep the vehicle from being "hijacked", because it looks kinda odd for a droned vehicle to suddenly change owners before it dies.