Just wondering if it's possible to add hard buttons to the Xp Plain. i.e. are there solder pads already in place?

No, there are no pads or test points for the buttons. I didn't plan to use them, I wanted to use a 2 sided PCB, and I wanted to maximize the ground plane on the bottom side./\r\n/You could add a wire directly to the micro, but since there is no display, you wouldn't know in which menu you are in.