What is exact use case? Adding 'Workflow Super User' (cq.workflow.superuser) at  enable the users of the group complete access to Workflow Models/Instances.
I have created a super user under an approver group and w/f step would be assinged to one of the users (approvers) in the group. I have added the super user in Workflow Serive configuration.
I logged as super user(approver) after the the author sends for approval, but I don't see this item in the inbox of super user.
The use is that the super user will see all the tasks assinged to any users under a group and would be able to work on those tasks.