I'm not sure that it is doable. The sprockets will not fit a drum hub or Nexus roller brake without some custom machining. (Manic Mechanic may do it)
Adding a rear disk might be easier. The Tophat adaptor allows you to attach a disk rotor and drive sprocket to your rear wheel (six bolt disk hub). An A2Z adaptor will allow you to mount a disk caliper to most frames (I have one on my pedal powered mountain bike). Venice sells caliper mounts and so does Chainganglowrider.com
IMHO, it is better to put your money into a drum or disk brake on the front. Coaster or rim brakes are adequate on the rear wheel if you have a good brake on the front wheel. It is far more expensive and problematic to put a disk or drum on the rear than the front.