[Kristoffer O]

A simple but not entirely statisfactory way is to use an existing site.

Horses Vale allows recruitment of Horse brother and horse brother commander. It is a unique site.

Remake the horse brothers and you have a unit and a commander linked to a site.

PDF used this in his seleucid-arco-mod.

There are probably other sites that are equally useful. Lost vale of the elders (gnome). Castle arcanum (warriormage nbr 2). The trick is to find a reqruitable unit that is not used in the game otherwise. A troll pit would be bad as summoned trolls would be subjected to the mod, not only those recruited in the pit.