Inco terms and payment terms in PO should come from PIR and not from referenced contract.

I am working on a scenario, wherein - The requirement is - when converting a PO from a Global Contract we should be able to use the Inco Terms and Payment terms of PIRs to flow to the PO superseding the ones that are maintained in the Contract. We are maintaining a central Purchasing org for the same. ITS URGENT PLEASE ASSIST.