I couldn't see another thread on this topic but found the information below in my search. I have since tried this on my PDA and it works, as long as you creat the UNL file to match the map name.

Up to four maps can be used on the Garmin software + world map

Valid map file names are as follows:

(world map) gmapbmap.img (reserved for world basemap)
(map 1) gmapprom.img
(map 2) gmapsupp.img
(map 3) gmapsup2.img
(map 4) gmapsup3.img

Hope this helps