You cannot add an item and purchase order combination that does not exist to a Vendor Bill.

AI generated answer (using internal Slack threads):

In NetSuite, when transforming a Purchase Order (PO) into a Vendor Bill, all items and quantities from the PO are included. This is because it's possible to pay for the entire PO before it is fully received. If you want to bill only the items that have been received, you would need to transform the PO, then query the item receipt data to remove or change the lines on the Vendor Bill. Disabling the 'Bill in Advance of Receipt' preference can also help with this. There is no standard field to link a PO to a Vendor Bill. If you create a Vendor Bill from a PO, the link is created. However, if you create a Vendor Bill not from a PO, there is no native link. Vendor Bills do not use the 'Created From' field in the same way as Item Fulfillment from Sales Order. It's possible to have several POs linked to one Vendor Bill. Manually linking a PO with an existing standalone Vendor Bill is not possible. You would need to delete the Vendor Bill and create it from the PO. Even with scripting, it's not possible to link a PO with an existing standalone Vendor Bill. The PO must be "transformed" into a Vendor Bill and this can't be done after the fact. ****** Disclaimer: This response was generated by an artificial intelligence assistant based on previous conversations from multiple Slack channels, and may contain inaccuracies or mistakes. ******