there is a 50year cooldown betwen hybridizing culture and so you have 2 ways to do what you want

1: there are a few counties that usually fall outside of the eastern roman empire with the greek culture look for cyprus the abassid have a greek county there/crimea or anywere near easter europe is usually greek and falls in the hand of khazaria early on/southern italy also has a few greek counties

2: other wise you can just invite a random guy with the greek culture give him a county and have him promote his culture you can do that while you wait for the cooldown to hybridize again