I would just buy an external bluetooth receiver and plug that into the input of the Crack. While it is possible to add such a module to the Crack, a lot of the ones out there will be expecting to be powered by a 3.7V battery, and you'll need to cook up your own power supply that's well regulated and well filtered to emulate that. Doing this with the power transformer that's in the Crack isn't impossible, but how one would accomplish this will depend quite a bit on the specific board used and how the grounding is done.