You can´t add new factions and you can´t change the cultures of those factions present either, the only thing you can do is changing the look of it. However, some things are culture-bound, such as the leader and event pics, or the culture penalty. As an example,you could make a second Egypt culture by replacing all, say, Greek cities units with egyptian ones. That that "new" Egypt faction, however, will retain the greek-looking leader and event pictures, unless you change those, too, in which case all Greek cultures will get Egyptian pictures.
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