Credits Tab: How to set Custom List for Role?

On the "Credits" tab of the "Record Properties" dialog box, the "Role" column contains a predetermined list of entries. I would like to change this list and add new entries, but this seems to be impossible.

The "Database" tab on the "Preferences" dialog box shows a list of "Custom lists," but this does not include one for "Role."

Am I missing it or is it not possible to change that particular list?

Thanks for any help.