If you open an MM file with a text editor you'll see that it has a fairly straightforward XML-style (though not strictly compliant) text format. The simplest route might be for someone to look at the Python script in the resources folder of this (.scptd script package) exporter, and add an MM writer function to it.

(Not a format that I personally use, so I'm unlikely to get around to that myself).

In the meanwhile, there are plenty of packages which read OPML and write MM so you can always start with an OPML export, and then re-export from something else.