I noticed that some mods have added cities and units to the BIC file, like TETurkhan's. How is it done?

Hit space or go to Map---> set player. Once a player is chosen, 2 buttons for cities and units respectively will become ungrayed.
Obviously this will only work for custom maps with custom player data.

Also, I'm not sure if thats true for vanilla civ, but I'm fairly sure it is.

Thank you GRM7584.