UPDATE: Aparently it was a GpsMapedit Bug in version 1.0.66.7 that allow to open NT maps packaged in gmapsupp.img, if you use older version it won't work, and if you use the new version 1.0.67.2 it won't work either.

There is a way. It only works with GpsMapedit 1.0.66.7.
Open the map in the Mapsource software from garmin. I suppose your map is indeed a junction of smaller maps.
Now, select at least two maps with the map tool, just put an sd-card with a card reader in the PC and export the selected maps to it.
Now you will have a gmapsupp.img in the sd-card, it is a package containing various img maps.
Now you can open it with Gpsmapedit 1.0.66.7 without problems, and select the map that you want from the list.
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I didn't know that. Could be interesting to some!