Well, technically, yes. BUT it's a L-O-N-G walk that will take you from the front entrance of Epcot all the way to the back (exit in WS), then walk over to the boardwalk and at that point you can either take a boat to MGM, or continue walking - it's about a 10 min. (3/4 mile) walk from behind BW to the entrance of MGM. Plus you'll need to enter Epcot and of course, it would have to be during the hours that Epcot is open.
Is there a reason why you want to get to MGM this way, vs. an easier way?