To be certified as a Nurse Massage Therapist, you must have your RN first and then complete 500 hours of additional massage therapy training through an acceptable school (I'm not sure what accreditation status is required). There is no clinical experience required beyond your basic RN training, although I would highly suggest trying to find a hospital that has an inpatient massage therapy program for hands-on practice in that environment. It's VERY different than the typical spa-massage training you get in most massage therapy schools. There are some "medical massage" programs now available that are more clinically based, but those programs seem to lack some of the quality of touch/energetic/intuition development work that is also important in massage.
Check out the National Association of Nurse Massage Therapist website to more details. I hope to join their ranks one of these days....... http://www.nanmt.org/about.html