(Ignore the thread date, the last update of the add-on was Nov 2013)
You could import from that to PHP BB and then from PHP BB to XenForo.
Note that some data is lost during the import. Whether there's a similar importer out there for other software that doesn't lose as much data, I don't know. But it's a good starting point. Look at the software you can import into XenForo and then see if there is an importer from Forum Motion to those.