There's not a translator available and MODX doesn't read the files.  

In addition, there will be Performances the MODX just can't replicate:

1) Any Part that uses AN-X

2) Any Part that uses more than 8 elements

3) Some limitations if you use more waveform memory on the M vs MODX's limitations (which are different between the plus and non-plus models).

The best you can do with what I know that's available is to publish your Performances from the M on Soundmondo (private if you wish).  Then use a MODX to load these up and finally export a user file and/or library using the MODX so you have used for this.  Anything with custom waveforms, custom (user) control curves, custom user microtuning, etc won't translate using Soundmondo so there are some limits beyond the 3 stated above.

If you have "vanilla" M Performances that could have worked verbatim in a MODX then this Soundmondo route can work.  

The worst case would be to sit down in front of MODX and manually create everything.  Obviously not the preferred or suggested route.

Create your performances on the Modx and then move them to the M. It won't work to try to do it the other way.

Generally speaking you need to do your core work on the OLDEST tech platform you plan to use. As p.80 of the M Operation Manual shows the M models are HIGHLY backward compatible with older models:

File formats used for older models
MONTAGE (.X7A, .X7U, .X7L)
MODX, MODX+ (.X8A, .X8U, .X8L)
MOTIF XF (.X3A, .X3V, .X3G, .X3W)
MOTIF XS (.X0A, .X0V, .X0G, .X0W)
MOXF (.X6A, .X6V, .X6G, .X6W)

Understandably it just isn't possible for a platform to be 'forward' compatible since it can't know what future formats might entail.

There are many other feature-related issues above and beyond those mentioned by the other responder.

You might ask around on the John Melas site if their software can move things around between the M and older models but I'd be surprised if they offer much in that area. They might be able to offer the ability to export an M performance as a Montage or Modx performance format but you will need to check with them.