If maps are made same for navmen, means they would more that likely be same format.
See if I had a GTS I would do this for you, probably in a day, but since I don't, all you get is my thought process in how I would get to the answer
Now if you find that format and get a new map, stick it on a sd card, and put it in to the nav unit, there has to be a map slot somewhere
if not there is map update option somewhere in software using the standard sd/USB slot. It's either that or it takes maps of the DVD/cd (less likely as this is old method circa 05~)
There has to be a way to update maps, be it under setting in the unit, or starting the unit in update mode. I can't test any of this, but the method has to be simple, remember, dealerships have to do this so it has to be doable by a 10 year old :p
Edit
So there is a SD card slot!
What you mean you couldn't duplicate it?
Did you view it on the pc with view hidden files/folders? There will be a file/cluster that id's it