CA_BI_INTFC Bill Plan PO Ref is not populating BI_HDR or BI_HDR_EXTRCT only populates the Bi_extrct_

We want the PO Ref from the Bill Plan on the invoice.  While we may place the PO onto the online invoice with no issues, we find that we need to look to the billing  Extract proj for the PO Ref

The PO ref is not populated on the Billing Header or Billing Header Extract table. 

Is there a way to easily get the PO from the Bill Plan to populate the Billing header Extract so the invoice would automatically pull  ? 

If we used the bill-by-id method, what would be the consequences ? 

Thank you in advance for all your help