Assinging different PO Approval Route by each Order Detail Line

My client is requesting to override the PO approval route code for each PO line.

The purchase order approval route is determined by a combination of the Order Type and Branch plant, User Profile, or Organiser AB no etc.

When I set up the approval processing option of the order entry program to retrieve the approval code from Branch Plant, the system populates the order approval route code from Branch Plant Constants. Then the approval route is copied from the PO header to PO details

However, I cannot find any function which enables Users to override the approval route code, or a flag which populates the  approval route code based on the BU (Branch Plant) in the detail line.