However, it might just be possible to replace a zone from one game with a zone from another. I think it would take a lot of tweaking, because there would be other references to other resources that would all have to fixed up correctly, plus the 'border crossings' would all have to line up, but with some work it maybe, just maybe, could be done.

If you imported, for example, an MM7 zone into MM6, your peasants would look like MM6 peasants and you would have to replace the existing monsters with some equivalent MM6 versions. Otherwise I think it's possible. A zone that is surrounded by water, like Mist or Eel infested waters would probably be the easiest to deal with.