I don't see why it cannot be done. The existing functions are there. Just relocating the function of the left stick movement (forward travel to fire gun) to a momentary pushbutton should be pretty feasible.

Once that is accomplished and now that the left stick's forward travel no longer fires the gun, it would seem that making it raise and lower the gun as separate functions, per your illustration, might be a bit trickier. In stock form the gun continually cycles up and down so you would need to be able to reverse the elevation motor.

Good ideas there,
-Harq