To be able to build them in a campaign you need to edit your export_descr_buildings file as well, for example for Companion Cavalry:

circus_maximus requires factions { carthage, parthia, macedon, seleucid, roman, greek_cities, }
{
capability
{
recruit "greek royal cavalry" 0 requires factions { seleucid, macedon, greek_cities, }

Watch the spelling, spacing and especially the comma after the faction name, even if it's the last one.