you can't combine two tables into a single report really without a lot of work such as virtual lists.
What might be acceptable without knowing your use case is making the report based on the same table your portal data is from, then showing the related data from the parent table in the report. This way you will guarantee seeing all of your portal records, and be able to show data related to the parents.
Basically, reverse the context you are currently trying.
Awwww, Mike... but Virtual Reports and SQL are soooo much fun!