You cannot, sorry. Not using SQL, anyway. Tableau isn't an ETL tool in that respect.
If you are using 10.0's cross-database join capability, then you should be able to "ask questions" of this joined data, but you'll do so via Tableau Desktop "dragging and dropping" vs. writing additional SQL.
Perhaps you can explain what your end goal is? It might help us come up with a better solution.
Thanks. My question was indeed whether I can use Tableau as an ETL tool. Now I know that I can't. It was just a general question, not for one specific problem.