To create a query that uses another query as a source 'table', just start a
new query. The Add Table dialog has a tab for tables and one of queries, You
can add the query that way.
That's not actually a subquery. The term 'subquery' usually refers to a
SELECT query statement that is part of another query. Here's an
introduction, with several example of how and why you might want to do this:
Subquery basics
at: http://allenbrowne.com/subquery-01.html