Was trying to figure out the same thing

not yet

I've been trying a couple of things but no luck so far. The best you can do is get the in game editor and change it manually.

If you move around what team you are on, you get different spawn points. Aside from that im currently not sure.

id say this is the only reason i dont use the map. i like playing as Rohan.. or i would, except i literally keep spawning in places without horses. i have one mod that created a scenario world based around TES world (Tamriel) and every civ starts where they are actually located in said world.
i hate to critisize, especially since i wouldn't even know how to start on a mod like this, but perhaps the author of said mod might be able to help, see how they managed it and do it here.
i really love the dedication put in, but the fact that i get Rivendell in a desert and Moria in the plains, Erabor not even remotely close to a mountain and, as said, Edoras without any horses kinda makes the map not worth playing.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=80443904
the mod i was referring to ^