Has anyone figured out how to create new Auxilia definitions?

Meaning, I'm well aware of how to assign existing Auxilia zones to new units - say, to make core Roman troops only recruitable in the predefined "Italy" zone - but where are those zones defined? I've tried simply adding them to the region_resources file with a descriptive string, but that doesn't help. So if I wanted to create a new Auxilia zone in Magna Graecia, how would I do that?