Rather than making a custom back as described at Wishlist:Expansion_Back, spacers could be used instead. They would be snapped onto the phone in place of the back, and then the back snapped onto the last spacer. The user could add as many spacers as desired.
Very crude artist's conception of a spacer:
One spacer being inserted between phone and its back:
One space already snapped into place:
Two spacers snapped into place: