Is it possible to set up a promotion modifier to add an item to a purchase order?

Hi Experts,

We have agreed handling fees with certain suppliers, which currently have to be added manually to the purchase order. I would like to automate this process and have the items added via a modifier in the Purchase Order.

I have now tried to set up a promotion modifier (as in sales) in purchasing based on the PO line, but so far I have had no success.

Thanks a lot for your tips!

Kind regards, 

CFE