You cant have routable maps in your unit covering the same region, if they overlap they wont show in gps and you'll have problems.
If a map is unlocked it doesnt need any .unl file. Every kind of .img should show in mylist if put in Map folder with anymame.img.
You can have CX maps , topo maps ,CN maps and select them in mylist .
With City Xplorer maps it's a bit different you can have CN and CX both in your unit,
The CX map is the same than related CN map provides,but you can have both in your unit and select CX in pedestrian mode when you want to use it, in my 3790 by the way, if i forget to deselect it , I can use CN just as pedestrian mode even car mode is selected.