I know this is a long shot, but is there any way to import MovieMaker files to iMovie? And I mean the editable project files, not the finished and flattened WMV or whatever...
Perhaps you can export the project in moviemaker as an xml file. You might also need to install perian (www.perian.org) so iMovie can play back the wmv files (actually, you might need to install Flip4mac to play wmv files). Then you could try to import the xml project file.
But honestly, I don't think that will work. You might need to start over (which might be better, because Moviemaker has crappy effects).