The brake light and turn signal are on the same wire, and connect to one of the filaments in the two-filament bulb. I'm thinking that new turn signal and brake wires will have to be run to the housing location. Brake wouldn't be too hard, probably by tapping into the hi-mount brake light. Turn signals might have to come from the front.
There's also the issue of the additional bulb sockets required, but that shouldn't be any more difficult to acquire than the housings themselves.