There is no way to do this, as the game is hardcoded at 8 Factions (7 playable Factions, plus the Native Life counts as a Faction) as well.
Also, even if this were mod'd in it wouldn't work very well, as there are other things to consider/ limitations of the game. A good example would be the Social Engineering table and how Factions respond to that: the game was designed to create alliances and vendettas based on the players' Social Engineering choices. If there were 14 Factions in the game, then there would always be several Factions choosing the same policies as the human player, creating a LOT of opportunities for tech trading, City swapping, EC lending, etc., all of which the human player can game significantly to their advantage.
D