I actually did this once. I made a map and totally rewrote the watermill .fhm file. Don't know if it's where you can do that anymore or not but that's just how I did it last...accidentally.
I'm currently at a computer without FH installed on it, but I'll give it my best shot lol
Previous of the update, you could open mapmaker and scroll to the very bottom of your list of maps and Watermill would be there, you could edit the terrain and objects and sky if you wanted and as long as it saved, the edited version would appear behind your login screen. Now, I'm not entirely sure as of right now if the watermill map can be accessed through mapmaker still, but give it a look see. Keep in mind, you cannot change the heightmap of Watermill.
If there's another way to add a different map, I invite anyone who knows to add on below, I myself would find it interesting to know how. Though, I believe I read somewhere that altering the map file itself for the login screen, that is adding a different map than Watermill, would have problems regarding the camera movement within the map, hence the disability to change the actual map itself.