I'm not really understanding the question because if you enter the "Advanced SetUp" menu from the normal "SetUp Game" menu, you can add extra AI players, adjust the number of city-states in the game, and make some other adjustsments to the "game rules" that will be applied for that game such as turning on Raging Barbarians.
You can choose to let the game select any AI players you add, or you can pick which ones you want in that particular game, or you can mix-n-match by directly picking some of the AI players and letting the game randomly fill in the rest.