Switch which civ you control. There's a button in the upper left corner of the main screen. Press it and you get a chance to take over control of any civ you have met, rather than your own.
This is Dales Civ Changer for BtS 3.13 made to work with BtS 3.19. I just winmerged the source code with files from 3.19, compiled the dll, assembled into a mod, and tested.
CivChanger for 3.19 2016-10-05
CivChanger for 3.19