I have spent many hours on a report and don't want to start over, so.... The report is table-based and a user wants one more field added. Sounds simple, right? I have a query that returns the value I need for the new field. How do I add it?

If you're saying that query returns this single field (as opposed to everything the report needs), you'd have to use a DLookup() in the control source of a textbox to retrieve the value.