I've tested and it works.

I wanted to "clon" the barbarian culture, just to have a second barbarian culture but not the same than the first (just to have some penalties).

I've copied the entire bi\data\ui\nomad folder in data\ui of RTW.
It was necessary to add the cards for new buildings.

But the rest was not so simple. I've modified several files, namely:
descr_banners
descr_cultures
descr_offmap_models
descr_settlement_plan
descr_sm_settlements
export_descr_ancillaries
export_descr_buildings
export_descr_character_traits
export_descr_unit
export_buildings (in data\text folder)

I imagine that it will be also necessary to modify several descr_sounds_* and export_descr_sounds_* files.

@kleeman
I didn't want to use that system just to be sure that the mod will work for everybody, irrespective if they have BI or not.