Defaulting supplier item number to a manually created PR and PO lines

Hi,

How can you default supplier item number when manually creating a purchase requisition or purchase order?

Here are the given informatio: 

(1)  all items have ASL setup wherein the supplier item number field is properly filled out. 

(2)  PO: Automatic Document Sourcing profile option value is set to Yes at the site level.  Automatically creating an approved purchase order from approve purchase requisition where a source document is attached is part of the business process implemented.  Setting this to No has affected this process.

(3)  Not all items have a source document.

Thank you for the responses.

Jacques Lao