In Apsona's single-step reports, can I reference a field in a calculated field without adding it as a column in the report?

No. In Apsona single-step reports, calculated fields can only reference fields that are included as columns in the report. The calculation uses the column names as variable references, so the field must be part of the report's data. However, if you don't want it displayed, you can hide the column after running the report, just ensure it's still included in the initial setup.