I did something similar for Carthage -
http://www.twcenter.net/forums/showt...v1-1-(Updated)
The table you are looking for is /db/diplomatic_action_subculture_restrictions_tables/diplomatic_action_subculture_restrictions/
Change the can_make_confederations value to True for the greek factions.
That will allow the Greeks to form confederations. However, their name won't change (and neither will their colors). I am not sure where that would be affected. I have tested this with Athens and was eventually able to get Sparta into a confederation, but I kept the name Athens.
If you want to make it easier to get a confederation, you can modify how friendly the greeks are to each other in the cai_personality_cultural_multipliers_table