Once you hold feudal provinces you want to culture-convert, build a tribe and pillage the feudal holdings.
You have to be very careful with this, because you're exploiting a bug. (EDIT: Also, you need to pillage the feudal holdings then build a tribe - not the other way round!)
The county must contain:
- 1 non-tribal holding, personally owned by you, which has at least 2 buildings
- At least 1 other non-tribal holding, which must also be personally owned by you
- No tribal holdings
- No holdings that are owned by anyone else
Then (order of steps is important):
- (Optional) Put a largeish army in the county
- Start pillaging the non-tribal holding with 2 buildings (1 or 2 buildings will be destroyed, but the holding will not be destroyed)
- Start pillaging the other non-tribal holding(s)
- Wait
- When pillaging is complete, all holdings in the county will be destroyed.
- Because there are no holdings in the county, it will flip to your religion+culture.
(Tribals cannot pillage tribal holdings, and they are not permitted to
start pillaging the final holding in a county. However, pillaging doesn't stop when there's only one holding remaining, which leaves a small loophole.)
How about tribal provinces? And nomadic ones?
Nomadic counties receive auto-generated tribal holdings when they are conquered, so the problem (and solution) is very similar to that of tribal counties.
You will need to make the county feudal (the steps below can be completed in almost any order):
- Give the tribal holding to a feudal vassal
- Ensure that the feudal vassal can't give the holding away (eg.
- Build the walls etc
- Convert the province to your religion
- Wait for the feudal vassal to upgrade the tribe to a castle
- (If necessary) Build a second holding (a temple)
- (If necessary) Build 1-2 buildings in one of the holdings
- Revoke all holdings in the county
Now the county fulfils all of the conditions for tribal pillaging, so you can follow the steps described above.
(Yes, this is very expensive and time-consuming. However, I'm not aware of any other way to do pillage tribal counties to the ground as a tribal. So, I would recommend you only do this for counties that are extremely important!)