Ben_jk,
here's how I do it: on your PC, send maps to an SD card using MapSource. You may select several different maps in Mapsource.
Copy the .img file to the folder Map in Android/data/com.garmin.android.apps.mobilext/files/Garmin/map (my phone is a HTC ONe, it might be a different location on your phone).
Of course you can get it off a Garmin GPS as well.

change the name in gmapprom.img