How to set Postgres tables to use columnar storage using dbt?

Solution 1:

TBH, I don't have any experiences with dbt.

However, given that dbt docs state that they support PostgreSQL, and Citus Columnar is a table-access-method level abstraction that doesn't require any changes to the queries(*), I wouldn't expect any compatibility issues when combining both.
My guess is that after creating your tables with USING columnar option, then you can just follow dbt docs for the rest.

(*): You might still want to have a look into that page to see what Citus Columnar does or doesn't support atm.
https://github.com/citusdata/citus/tree/master/src/backend/columnar#introduction