I'm working on a DWH project using Oracle

I have a compressed table (dimension) which has about 35M rows called CUST (customer). I got 15 tables, with an average of 100M rows, having a FK against the PK of the CUST table.

What I have to do is to add 2 new columns in my CUST table and they will never be null.

So my question is: which is the best way to add the 2 new columns.

Should I add the columns and then update the entire CUST table.
Should I create a new temp CUST table including the 2 new populated columns, disable the FK constraint realted to the CUST table, truncate the CUST table, insert into CUST the values from temp CUST and reenable the FK constraints?

Would be grateful for any advice