There is a way.... I found by mistake...
You do need a fully unlocked WP7.5/7.8 device, the XAPS need to be for fully unlocked phones(with nothing ripped from them).
The key to this is, you MUST download the demo of the game and play it once (this will show in your history), then install your XAP, you will notice Achievements will Pop and they will show up in your Xbox Achievements...
Your welcome !
PS. I would be careful with this, as if you are nabbed for Cheating on Xbox live in anyway, you get reset to a -0- gamer score. I have seen this happen to people who found a little loop hole someplace to get tons of points.