Look at the frame at the top of the forks and seat stays. If there is a hole in the cross member you can mount caliper brakes.
Check the link below, the first reviewer said it worked on his cruiser.
http://www.amazon.com/Star-1000A-Caliper-Brake-Silver/dp/B0015S4L7E/ref=sr_1_2?ie=UTF8&s=sporting-goods&qid=1265754617&sr=1-2
http://bicycletutor.com/part/brake/
Check the link below, the first reviewer said it worked on his cruiser.
http://www.amazon.com/Star-1000A-Caliper-Brake-Silver/dp/B0015S4L7E/ref=sr_1_2?ie=UTF8&s=sporting-goods&qid=1265754617&sr=1-2
http://bicycletutor.com/part/brake/