EDIT: 24 FEB 2009 - The suggestion to use GMAPSUP2.IMG works just fine. So to confirm Iron's suggestion, you can have four map files in the Mobile PC folder: GMAPBMAP.IMG (basemap), GMAPPROM.IMG (preloaded Mobile PC maps), GMAPSUPP.IMG (compiled mapset from Mapsource), and GMAPSUP2.img (another Mapsource compiled mapset).
Your suggestion will create problems for someone with Mobile PC.
GMAPBMAP.IMG is the name of the basemap that comes with Mobile PC. If you use that name for the detail map you will delete the basemap. That's not a good idea.
GMAPPROM.IMG is the name of the detail maps that come with Mobile PC (North America or Europe). If you use that name for another detail map file that you create with Mapsource, you will delete the detail maps that came with Mobile PC. That might be a problem for many people who want to still be able to use those maps.
I'm not sure if Mobile PC will do anything with a GMAPSUP2.IMG file. I have not tried that so I will.
If you want to use Mapsource to add multiple maps for use in Mobile PC it is still best to select all of the maps you want to use at one time in Mapsource and let Mapsource compile them into a single GMAPSUPP.IMG file. That way you can use Mobile PC to switch between them.
If you are using them on memory cards and they are too big to put on a single card, you have to use multiple cards anyway, so just split them between cards in the best way that works for you. You have to change cards to change maps anyway, so the file can be named the same (GMAPSUPP.IMG) on each card.