The AI Governors seem to be set in GalCiv3AIDefs.xml this time round. And with no available option to add commands into the xml's by editing the Schema's, replacing a Governor is the only option.
As to the player Governors its possible to add new ones. But again I think you would have to replace a default Governor since there is no way to add new UI elements to the game either. TBH I have not looked to see if just adding a new Governor updates the UI in game or not, I'll check in a bit.
EDIT:
Yup the UI auto updates if you add a new Governor, but I think it replaced the Balanced Governor in the UI screen?
