Blanket PO's and multiple supplier sites

Summary

Can a BPO be defined that applies to all supplier sites within 1 business unit

Content

We have a requirement where a blanket agreement is wanted for a supplier, and that it applies to multiple supplier sites within the business unit.

I know that you can enter a BPO without a supplier site in the header, and in the Controls tab, you can enter a purchasing site for each business unit.  But you cannot enter multiple purchasing sites for the same business unit.

The BPO's are used in Self-service procurement, so this is a consideration also.

Can this be done, or do you have to create a BPO for every supplier site.