To query on a query you can do the following:
  1. Create a query (see How do I create a query in Raiser's Edge (includes video demo))
  2. In the query, go to Tools > Query Options and select the Record Processing tab
  3. Mark the box for "Select from query" 
  4. Use the binoculars to search for your existing query
  5. Click OK and continue with building your query