Description based BPA - PO auto created - Supplier item mandatory?

I have posted the questions here.( https://community.oracle.com/customerconnect/discussion/760286/agreement-not-coming-up-in-lov-while-creating-requisitions-that-are-description-based#latest). However, I was asked to post this specific question mentioned below in Purchasing forum and hence posting the same again here.

1. The purpose of creating a BPA as an purchasing document out of negotiation outcome is defeated if the customer has to go and update the supplier item after it is approved. Can you please share some thoughts on the same or any workaround to avoid this. This becomes cumbersome especially with BPA having more number of lines; manually updating supplier item for each line becomes a task for customer. Also, during negotiation creation for BPA as outcome there is no field to enter supplier item during negotiation lines creation. So, cannot add this during negotiation creation.