How do I add extra civs to scenarios?

[SuperClass1]

I have had civ4 BTS for a few months now and i have decided that the earth 18 civ scenario is my favorite. I've started to get into a little bit of modding and have added extra resources and terrains through the world builder to all the civs to balance it out between every nation and to make it more interesting. However, the earth 18 civ scenario only has the civs from the original game and I cannot find a way to add the extra BTS civs in the scenario through the world builder. For example, I want to add the Zulu in south Africa, the native Americans in North America, the Korean nation in Korea, and so on. I have spent a good while trying to find a way and have come to a dead end. Surely there must be a way to add other civs in the scenario. That scenario would be a lot more fun If I could add all the BTS civs in it. Please respond if you know how to do this. Thanks!