afaik we can't add new CS during game, but we can force the game to load all 41 CS then "kill" them at first turn (delete units) and "resurrect" them when needed.
I use the same method for major civs (load the 22 slots, kill the units of the civs that should not start at 4000bc, then create new units for those civs at specified times), and same for the player civ, except that if he use a civ with a delayed start you need to keep one hidden unit alive on an unreachable part of the map (and force a kind of auto-turn).
I use the same method for major civs (load the 22 slots, kill the units of the civs that should not start at 4000bc, then create new units for those civs at specified times), and same for the player civ, except that if he use a civ with a delayed start you need to keep one hidden unit alive on an unreachable part of the map (and force a kind of auto-turn).