Garmin C550 Street Pilot - More Maps?


I have been searching for maps that I can load on the SD card in my unit with out spending several hundred dollars. There is a Garmin Mapsource CD for World Maps on EBAY that according to the Garmin compatibility chart, will work on the C550.

If I got one of these, would I be able to copy map sections, like for example the Pacific Coast of Mexico, and load that on the SD card? Then would the C550 show this as an additional map selection and work with that map like it does with the pre-loaded North American map?