Factions.xml for classic EAW
and additionally
Expansion_Factions.xml = Underworld Faction for the FOC Addon
You could try to download the pegasus chronicles and get from there
Factions_Additional.xml = Many other costum factions
then overwrite original Stargates FactionFiles.xml to initiate the 2 new files there.
I guess it wont work by the first try, maybe if you play with it you could add a faction but this is hard to achieve.
Also you can open any GC XML File ( indexed in CampaignFiles.xml ) and search for <AI_Player_Control>
This tag assignes a faction to a player. SandboxHuman is the human player, rest is AI
So if a faction allready exists you can assign any AI to it like for example:
<AI_Player_Control> Underworld, SandboxHuman </AI_Player_Control>
<AI_Player_Control> Rebel, BasicEmpire </AI_Player_Control>
<AI_Player_Control> Empire, BasicEmpire </AI_Player_Control>
<AI_Player_Control> Pirates, None </AI_Player_Control>