What you need to do is. Go to the export_descr_unit file, find the unit you wish to edit. At the bottom of the units info it will say something like:

ownership macedon,

you want to change it to

ownership macedon, greek_cities

Now you want to go to export_descr_buildings. Find the stables section, and find the appropriate level stable, find the greek lancers entry.

Change it from:
recruit "greek light lancer" 0 requires factions { macedon, }
to
recruit "greek light lancer" 0 requires factions { macedon, greek_cities, }

Don't forget the comma at the end of greek_cities. I haven't actually tried this, but this is exactly what i've done to add companions to the greek cities, the only problem is you don't have a unit card in combat.