But there is some distance you can go towards what you want to do...
Easiest.. open two mapsource sessions... Set them to full screen position and zoom them both to the same area... (drop a couple of corner POI's onto both maps and use them to make aligning the two sessions easier).and alt-tab quickly between them... not great...
You can also (whispers) use the same map making tools that created the camera transparent map, to open the garmin maps in the area you want... and merge the two... When you re-assemble them, you will have lost a ton of the original data, but it will still look like a garmin map with the new POI's on it... This option involves several evenings study to learn the tools...
And I've just thought of another... Go to the source that provides the camera database that is for GPSR's that have waypoint warning... ie they are just a big file of proper waypoints.... (as opposed to the transparent overlay one) and load that while looking at the map and area in question... You could even load those waypoints, while looking at your overlay map and see if any are missing in the areas you care about...
Hope this makes sense, it's getting late here...
Al...