Adding item in Purchase order once submitted

I need to add a row to a purchase order that has already been submitted, but there is no box to assign the new row to a project.
I can get the new row added with item code, qnt and price, but I can’t assign a project so the changes don’t save.

If you do that, ERPNext will behave badly!

Submitted documents are documents supposed to be freezed and ready to archived! So that’s why you shouldn’t edit an submitted document!

The way to go, is cancel and amend the document!