Unlike most garmin devices, MapThis uses image tiles (not vector maps). so any of the vector formats that Garmin usually offers would not work directly.
If you find a way, to convert your garmin maps to a set of png/jpeg images (for the area that you are interested in), you can them use MapCruncher and gmdl tool to convert them to a format suatable to mathis and align it with gps data. You can find more details by searching GMDL thread in the new forum.
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