Steps Beyond Foundation continues its Artists Talk series with "A Conversation with American Dance Machine 21," featuring directors Nikki Feirt Atkins and Margo Sappington, joined by Robert La Fosse, Tome Cousins, and Lars Rosager, along with ADM21 dancers Georgina Pazcoguin, Amar Ramasar and Jessica Lee Goldyn.   Atkins and Sappington will discuss their reactivating the American Dance Machine, inspired by their commitment to preserving the heritage of musical film and theater choreography.   The evening will include dance clips from the Company's recent season at the Joyce, and conversation about reconstructing the great dances.   ADM21 will also hold master classes at Steps on Broadway, March 16-27, 2:30 to 4:30 PM, with La Fosse, Cousins, Rosager and Lynn Sterling teaching dances from On the Town, Jerome Robbins Broadway, and more