seem to be quite solo here, but I found a solution for the last above question myself.
Maybe there is a more elegant way, but that's how I managed it:
My goal was to get a transparent City Navigator NT map on top of a Recreational Map in order to see topo features while navigating.
My first (failed) approach was
- to create 2 seperate temp_x.img files (1. City Navigator, 2. Topo Map) each covering the same region (added file size <512MB)
- then use gmaptool to set the City Navigator temp file to transparent
- then upload both temp-imgs together to the 276c's data card with [Only registered and activated users can see links. ] or [Only registered and activated users can see links. ]
but obviously it is not possible to upload img's from gmap coded maps this way

second (successful) way was:
- set a previously created City Navigator gmapsupp.img transparent (with gmaptool)
- convert this file to 'old Mapsource format' with [Only registered and activated users can see links. ]
- now having both maps installed in MapSource it was easy to select a map set containing desired parts from both maps and transfer them to the data card the conventional way.
MAP LOOKS GREAT! :-)