This one is in response to an ongoing discussion about the pressurized volume in the various deep space exploration architectures. Apparently an Orion will provide sufficient crew volume for extended missions while a Dragon will not in many peoples minds. The suggested often remedy is a Bigalow inflatable module for the extended missions.
Henry Cate has a cheaper and faster solution. The trans-Lunar/Mars/asteroid injection stage has a large hydrogen tank. Add a small header tank to the vehicle to store the residual hydrogen in the main tank and vent that main tank to vacuum. Fill the tank back up with breathable air and dock the Dragon to it as in Apollo/LEM. Requires compatible hatches designed into the tank.
The living volume gained will necessarily be Spartan for the same reasons that Wet Skylab was rejected. It does however provide at least 14 cubic meters of volume for each ton of hydrogen that was carried in the original tank. This in addition to the usable Dragon volume would quickly dwarf that of Orion without the launch or purchase expense of the inflatable module.