How to create a product purchase invoice using Incoming Document and OCR Service in NAV 2016?

For my customers, the majority of purchase invoices they receive are for product - i.e. have lines of type Item. I used the Incoming Document functionality to receive a pdf of a product invoice, then sent this to the OCR Service where it was read and details updated on the Incoming Document. So far so good. I then used Create Document, and a Purchase Invoice was created with a single line of type G/L Account. What I want to do is to use the Get Receipt Lines function to bring on the Posted Receipt Lines, which I could do - but I cannot delete the original G/L Account line, or amend it in any way other than by changing the G/L Account. So I thought about putting a negative line on to balance out the initial G/L Line, but, guess what, you cannot add a line on either, other than with Get Receipts, and you can't amend the quantity or value fields in the lines created by Get Receipts. Any thoughts as to how I can make this work?