Can games made with MMF ever appear on Steam? I mean, is it technically possible or does Steam require something that MMF games currently can't handle?
Technically I don't think there's a problem. You just need to include the cncs*-whatever DLLs.
I think there might be a problem in actually getting Valve to put your game on Steam. I would imagine there's some sort of requirement to how big your company is etc. like with Nintendo. People who want to create games for the Wii (even small time developers) are required to prove that they have a company with at least 12 employees and stuff.