Very pretty simple...

Just use the patched versions of mapinstall or mapsource to create your mapset (gmapsupp.img).

Either of those programs will allow you to save the gmapsupp.img output to any hard drive on your system.

Once it's on your hard drive you can do whateder you'd like with the gmapsupp.img.

You can use tools to modify it's header contents or unlock it and then just copy it do any data card or GPS you'd like.