Is it possible to populate an invoice with a custom field associated w/ a Project?

Is it possible to populate an invoice with a custom field associated w/ a Project?

I typically get assigned a PO from my vendor to do work for a specific project. I would like to create a custom required field "PO" in projects, and have that "PO" field displayed in any Invoice that is generated to that customer for that specific Project. Is this possible? if so, how would I do that? 

TIA!

Cheers