Can there be only 1 position inside a PO approval hierarchy?

  We have a requirement to have one position defined inside a PO approval hierarchy which would be then be assigned to a document type (standard Purchase order). A list of people will be assigned this position and they can approve a standard purchase order below a certain $ amount threshold.

The application doesn't allow me to define and assign a position as reporting to itself.

When i check the owner can approve checkbox and not attach a hierarchy or a workflow, Can't the buyer entering the standard Purchase orders also approve them?

We want to use standard functionality for this requirement.