In Assets\XML\Civilizations\Civ4CivilizationInfos.xml under CIVILIZATION_BARBARIAN, <Units>. That section normally specifies unique units, but, in the case of the Barbarian civilization, it just makes certain units unavailable. This can be undone by simply deleting <Unit> elements. However, only units that are able to attack will be spawned (by the DLL – CvGame::createBarbarianUnits).