It would depend on the use. If it's to be wired into the circuit inside the controller for pressing a button more rapidly or something, just wire it in. But if it is for another use, you'll have to break out a program to code the button into the microcontroller inside the Xbox (or USB?) controller.
Or if you mean repositioning the already existing d-pad button, then open up the controller, drill some holes, patch some up, and stick the button out of one.
uh... whats the point of doing that? :-l all you'd prolly have to do is find a seperate controller, take it apart and take the control stick out of the other, [they will be connected to a box inside] try welding them off o_o then put the d-pad on it.
I'll look later, but there was a guy on Ebay selling various modded controllers. The big one was the one that put the XYBA buttons under it so your fingers were always touching them. Sort of like that FPS controller that was out some time ago. You use your ring and middle finger on each hand to push your buttons, guy says it'll make your game better because you're not using one thumb to push four buttons, but four fingers for four buttons... The guy also mods controllers for other games, like R6...
I think the D-pad mod would be a great one to add, just so that you don't have to move your thumb off the stick to chat.