You can load extra maps very easily using the CartoType framework's LoadMap function. The only requirement is that the extra maps have the same projection as the main map. You can load extra maps using the Maps App, which has an option "Open in Current Map..." in the File menu. You can unload extra maps using the framework's UnloadMapByHandle function. To do that you need to store the handle of the map you loaded, which is available via LastMapHandle, a function to return the handle of the last map loaded. Maps that have been loaded can also be enabled or disabled using EnableMapByHandle. Disabling a map does not unload it.
Here's a map showing the world's coastlines using the spherical Mercator projection ("Web Mercator").
First I load a map of Greece and zoom in to show that it's really there:
and finally I change the style sheet and zoom into Shanghai: