Can you add a requisition line to an existing PO line?

• We have an existing PO with 2 lines.  Invoices have been matched to these lines for partial amounts and they have been paid.  We have a new requisition that needs to be added to the PO.  We need to add this Req line to one of the 2 existing PO lines.  My understanding is that we cannot do this for Complex POs, but should be able to do it for Standard POs.  Is there a series of conditions that need to be present in order for this to occur? 
• We tried to add a Req line to an Existing PO line and the Req line disappeared and the PO was not increased.