I copy and paste from a post i've read on Smellybiker's Wanderlust Worldmap --thanks Hans for your post
coz I think you can try :
I've got an European 2720 and I also like to change sometimes the primary map on my Garmin. Well... just this week I found a solution on an Dutch forum. It's so simple...
You need this:
An SD card (at least 2GB)
An SD card reader, attached to your computer
City Navigator NT Europe (or whatever map you want) installed on your computer
The MapID from that map
WebUpdater.exe (you can find in the Mapsource or Garmin directory)
Let's say I want to change the primary North American map in a European map. First thing to do is to make an imagefile from that European Map. Start MapSource, install City Navigator NT Europe on your computer and select all the maps. Put an SD card in your computer (maybe you have to buy an SDcard reader,I've got an build-in at my Asus Eee). You can now send the maps also to the SD card instead of sending them to your 2720. Send the selected (red) maps to you SD card. Now you can take a coffee of three, four, or go painting your bathroom. It took about an hour and a half on my computer to send the map to the SD card.
Take a look at that card when it's ready. You'll see that MapSource have make a folder on it with the name 'Garmin'. In that folder you'll find an image with the name GMAPSUPP.img
Rename that imagefile to GMAPPROM.img and put it in a map on your computer, let's say in C:\GARMIN
Open a DOS box and go to the map C:\GARMIN
Give this command: C:\Garmin\WebUpdater.exe /upload_file rgn=XX path=C:\Garmin\GMAPPROM.IMG -autorun usb /unlock_check fid=XXXX pid=X /i (sustitute XXX with your map FID values)
Again it will take a long time to upload the file to your 2720.
But... that's all folks! It's simple as that. After this you have a new map on your 2720. The GMAPPROM.img has overwrited the original one. And you have still plenty of room for other material. If you send some maps thru MapSource directly to your 2720, those maps will get the name GMAPSUPP.img and won't overwrite your just built primary mapping.