As Parky says, install to an SD card using MapInstall (PC & Mac) or Mapsource (PC). This will create a GMAPSUPP.IMG file in a folder named Garmin.
Using either of the above programs you can create map-sets from any mapping that you can load into them. You can combine different mapping in one GMAPSUPP.IMG file if you wish or have multiple SD cards with different mapping on each, swap them according to your requirements.
Having your primary mapping on an SD card does carry a small element of risk as one of the guys in the VFR club found out; Card went bad on tour in the South of France, he had to resort to paper maps and navigate a simplified route. Either carry a spare duplicate SD card or put the important mapping on the internal memory.