- My View
You would add the list field to the item being approved, not the approval record itself. The sysapproval_approver table already has a field that points to the user table. This approval is for that user - is what that record is for.
If you were to add a list field or variable to the request item, for example, you would need to ensure your approval activity on the workflow is aware of it and triggers the approval accordingly.
--Platform Architect - ServiceNow