hidden resources are static, while a building can be built (by choice, or by script) so for instance, requiring a building 'roman_culture' to build milites, for example, after capturing a new region for first few years ud be reliant on local auxiliaries but after a few years (when building is built) u can recruit the milites. (the local auxiliaries can also have a 'and not roman_culture' line and makes them unrecruitable w/ the roman_culture present).
the same can be true w/ barbarians, make them able to recruit these roman-like units only in a settlement w/ roman_culture.
This essentially makes the romanization, barbarization, slavicization, etc of regions possible.