it's not do able. You must flash the firmware of your optical disc drive (not the hard drive) to play "backed up" game discs.
Doing so will void any warranty you have with your 360. Which also means if your 360 ever RRoD's on you, you may not be able to get it replaced/repaird by Microsoft.
You'll be able to ship it in for repair, but if they see you hacked your optical disc drive, you're shit out of luck.