I don't think you can add eras and civs--the game will not handle it. I've tried adding a civ and the game crashed right after clicking "New Game" or "Quick Start." When I tried adding a future era after the modern era, the game didn't crash, but I could never progress into the future era, even after researching all the modern era techs. So although you can use the editor or the Civ3MultiTool to add things, the game won't necessarily work if you try to add certain things.