DirecTV button missing

It seems the X1 will not learn DirecTV commands from the old remote, which isn't too bad except there's a button I need that's not in any of the available devices from the could. It's the "real" back button. What says "back" is really "previous channel" and I want something that goes "back" in the menu system. How can I add it since the X1 will not learn from the DirecTV remote.