Barbarian settlement levels hardcoded to 3?

Another indice that this question is hard-coded is this text in event_strings.txt in Data\text.

Code: [View]

{upgrade_settlement_to_large_city_body_barbarian}		THIS UPGRADE CITY TEXT SHOULD NEVER APPEAR ON SCREEN AND SHOULD NOT BE TRANSLATED!

In the case of city level, the first time it appears the same, but at the end of the file it is changed (it seems that finally it was decided to give the barbarians up to city level).

I have no idea if the circumstances when the "events" appear can be modded.

Edit: another problem (in case it could be modded) would be that there is no barbarian_large_city and barbarian_huge_city defined in Data\settlement_plans folder. I don't know if it would be enough by copying and renaming some other (roman for example).