Custom columns are report or export fields that are derived from user-defined formulas. Users create these formulas by combining columns (or fields) available in the report or export with operators and/or functions.
Adding a Report
Only users with Add Public Reports permission can add new reports.
If you can't add a report, but you have Add/Delete Private Reports permission, you can add a new report by saving a copy of an existing report and then editing it.
Editing a Report
For any default report or report added to the system, you may be able to edit what fields and filters display, and change other display settings. You can edit the system-level version of the report or to your local version of the report only, if you have to correct permissions.
To edit reports, the user must have Edit Report Settings permission. To edit a public report, the report must be enabled in the user's permissions.
You can add user defined, custom columns to any report, but only when editing the report, not when adding.