There are currently no "mophie pack" type cases for the 925 (would be awesome if there were, using the wireless charging case pins!). You also can't fit a larger capacity battery inside, as it is surrounded by other internals and would not fit.
You CAN get one of those juice packs that are basically an external battery, plugs into your standard charging cable, and it charges the internal battery as if you were plugged into wall power. Down side is that you have to make sure that is charged, and carry two things with you.