You could save one of your games (worldbuilder) with Barbarians on the map or in control of a city and check the save game for them.
I have done that via the world builder and have saved the game there. In this file the Barbarian troops have "UnitOwner=18", so 0-17 are the playing countries and 18, as the number following the last country are the Barbarians. So you can do it with an editor.