You can attach one end of the tactile switch to the leads that go into the button, the the other end of the tacts to the ground, I'm not sure about attaching a piece of another controller to it though. This is the best I got for you.
It would be easier to just relocate the d-pad and keep the cpu chip on the side, withthe B and A buttons.The only differnce is which side has the rest of the motherboard connected to it.Also the number of traces is less for the D-pad.