Our ward is doing it right now for a Sister who moved just before going on her mission - we are supporting her mission, not her new ward. It is quite easy when the Bishop calls the Missionary department to discuss (should be the Bishop of the ward taking responsibility for the financial obligation, I think). However, it may not be so easy for other areas of the world. I am very interested if this could be done, though, as a member wants to support a missionary from his daughter's mission (Paraguay) and we are trying to figure out how.