Change definition of Player Sides in an extension

At present one can only change definition of Player Sides at module level. But it would be nice to be able to edit it at extension level, by adding extra sides.

For example to support a 5-6 players expansion to a 1-4 player game, which players can include or not, and if they do then Yellow and Purple - say - should be added as selectable factions to the base 4 colors/factions.

I realize that there would probably be a bunch of associated things that would need to be changed as well, though. For example, allowing to add at extension level the name of the extra player sides as sharing certain private windows (with allies, say) etc. But just allowing adding extra player sides at extension level would be very useful as a first step, I think.