You can do so today, as third parties sell cases for the phone with a battery in them.

I made use of one during my motorcycle trip. On the bike, the phone was plugged into power and sat in a form fitting mount. When I'd spend a long time in a city, I'd put the phone into the Mophie case. It provided the extra battery power needed to use GPS along with the camera most of the day. Both these functions can be big power drains on a phone. When I didn't need the extra power, the phone could go back to it's slimmer and lighter profile in my pocket.