how to enable source document field in purchase order line

Hi,

in 11i iProc catalogs were linked to CPAs and some times users create a non-catalog requisition for some items and miss to specify a CPA(Contract), so the PO created without any Source document. but Buyers were able to add the CPA to purchanse orders in core purchasing form.

But in R12 iProc catalogs are created as GBPA and Buyers are unable to add the GBPA reference to the purchase order lines, as the Reference document "Document Type and Document" are disabled (Purchase Order Form -> Reference Document Tabpage)

any idea how we can add the BPA/GBPA number to a Purchase Order after it's approved?