id have to look into it since i havent really moded planets/structures yet. however, a few files that i can think to check off the top of my head would be the .entity for the com center (to see how thats set up, and what modifyers it uses) the gameplay.constants, the actual planet entity files, ill see if i can come up with a more solid answer after i actualy take a look at a few files.

Edit: ok, well after looking through a few files i dont think its possible to make each strusture itself spread culture, howevery, you might be able to make an ability to do so and add it to the structure.

Each structure has a slot for 4 abilies, so you might be able to make an ability that increases the culture.

in the modifyer list theres a EntityModifyer "CultureSpread" that if used properly, might be able to do your desired effect. i dont have time to try this myself so you will have to test this yourself.