Custom Group Formations

I was wondering, are Group Formations hardcoded, or would it be possible for certain factions to have custom Group formations, such as the Roman quinqunx with the spaces built in, or the Macedonian style lines used by Alexander?

I suppose the problem could be in how the game defines missile, foot and cavalry units...

While I believe it MAY be possible to include new group formations, I'm almost certain that it isn't possible to make any formation(s) available only to one or some factions. Any new formations that might be added would have to be made available to all factions in the game.