easy peasy my friend.... if you can install spit fire you can do button relocation. Gameroms has his entire d pad discretly wired on bottom of one of his controllers.
the new version of Common Ground is esiest, but if you are only doing one button than it really doesnt matter.
may have to scrape the solder mask a little for soldering but essentially you just hook button up either side of the button pads that the rubbers in the controllers hits to make a button press. and presto you have relocated button and original still works, as long as you don't physically get in the way of rubbers.
Post what you want and what controller type you get, and I or someone can mark the spots on a jpg and post it for you.