Magic site modding: is it possible... (mod Absolute Ermor)

[fosforo]

I'm doing a mod: Absolute Ermor, focused on this nation.
My fists purpose was to rejuvenate Triari and Praetorian Guard (45, 48 respectively), then i gather enthusiasm: I've modded the apperance of velites, alae legionaries and princeps now very well recognizable; after that a beauty treatment for the Thaumaturg and the introduction of the new unit: Equites.

Now, I'd like to improve the role-playing depth of the other units and to make that, I nedd to to create a new magic site that:


1) is not unique but is searchable (easy, I can do it)
2) is not a starting magic site (easy, I don't set it in the nation starting site list)
3) is EXCLUSIVELY searchable from unit of an unique nation (what can I do???? )

I read two times the modding documentation but I didn't find anything to obtain 3)

If I create a site searchable, anyone of any nation can look for it.
In detail I like to create a magic site "Colosseum" searchble only by the Ermor units but that is not a strating site of the Ermor province; in detail, the "colosseum" can be appear only in province governed by Ermor.

Is it possible? How?
Thanks and sorry for english,
Fosforo