Did you try to put the map into the User folder that Home generates?
Here is how a directory containing an original downloaded map looks like.

Directory D:\Users\Username\Documents\TomTom\HOME\Download\complete\map\Nord_America

22/11/2009 19:39 <DIR> .
22/11/2009 19:39 <DIR> ..
27/10/2008 08:50 226 activation_map_50377.cab
27/10/2008 08:50 7,561 map9528-1.gif
27/10/2008 08:50 1,653 map9528.gif
27/10/2008 16:41 132 North_America-15.meta.dct
27/10/2008 09:08 3,479 North_America.toc
27/10/2008 09:08 2,000,437,301 North_America.zip
6 File 2,000,450,352 byte
2 Directory 25,230,520,320 byte available
D:\Users\Username\Documents\TomTom\HOME\Download\complete\map\Nord_America>

The TOC file is a XML file, pretty complex indeed.

But if you have already a map in your device you can back it up via Home, then you should find it below
D:\Users\Username\Documents\TomTom\HOME\FromDevice\map
This will give you the "structure" of a folder containing a map.

The idea is to create then a new folder, similar to the one you got via backup, which you should put the new map into, so as to have "Home" see it.
It's worth to try.