JaVaWa GPS-tools | IMGfromGPX
Try this one, it makes a version of the map that can be installed on your computer, that way you can use mapinstall to install it along with the other supplemental maps you need all at the same time.
Problem is if your gps only supports one gmapsupp file it will always overwrite the existing one.
That is a limitation of the gps, not the program.
What model gps are you using?
if it has a memory card you can put the maps from this utility in the gps memory and the other supplemental maps on the card.
O well just some thoughts, it is a nice utility and it works quite well.