Problem now is that my company's process requires the proforma invoice to be created before PGI is posted.
The above can be achieved through standard only (F5 order related and F8 Delivery related) system does not see the PGI status when you are creating proforma invoice this was already indicated by Santosh. No need to develop any new requirement and assign here.
However, if you want create a commercial invoice(with accounting generation) before PGI you need to change the copy requirement from 003 to 011 in VTFL in copy requirement field.
thanks,
Srinu.