I would simply use the export_descr_buildings file for that purpose and set the Thorakitai Reforms as the date for allowing such recruitment. In fact, I've already done that in my own version of the mod for Hellenistic factions who take Syracusae in Sicily, Massalia in Gaul, or Emporion in Iberia. I've also pretty much done what Pooploop is suggesting for Italy.

It is time consuming work, but fun to tinker around with recruitment and "what if" scenarios like that.
Koinon Hellenon also has decent recruitment options in the West with Supervised Native and Hellenic administration buildings, and even the Allied League factional government building as far west as Massalia. However, they don't have phalangitai unless you mod the game to allow them to have them, only the Deuteroi phalangitai in Greece and western Anatolia. Also, with foreign military garrisons and allied democracies/oligarchies any faction can recruit Camillan and then Polybian Roman units in Italy (minus Triarii).