WASHINGTON, Jan 29 (Reuters) - The John F. Kennedy Center for the Performing Arts is planning a $100 million addition, including an outdoor floating stage on the Potomac River, in its first major expansion since it opened in 1971. Three connected pavilions to house classrooms, rehearsal rooms, lecture space and other facilities were also included in initial plans for the U.S. capital's premier performance space, which were laid out on Tuesday.