How can we fetch additional data elements and print on Standard PO Document

How can we fetch additional data elements not available in "Standard Purchase Order Data Source" BI Publisher Data Definition and print on a Standard PO Document?

Does Oracle provide a Hook to place our own code and produce additional XML Data Elements?

We are putting information on "Note to Supplier" and "Supplier Item Number" fields on PO lines that we want to incorporate into a separate section on a PO Document. This section won't necessarily print per line and also may have to print Numbers calculated based on what is entered in those fields.

Has anyone done this?

On a separate note, has anyone tried entering PO Lines with Negative Price?