Procurement Cloud - Unable to add the newly created PR to existing PO once submitted

Summary

Procurement Cloud - Unable to add the newly created PR to existing PO once submitted

Content

We are facing the issue as below. Let me explain with an example. 

1. PR1 created. PO1 Created through PR1 using Document build feature. Now PO1 is in open status.
2. Now for business reasons, we need to create PR2 under the same supplier and add it to PO1. 
PR2 should come as second line in PO1. 

But we are unable to add it to PO1. Because in Add to Document Build feature, for PR2 if we search PO1 its invisible.

It would be great help if some gives the workaround or solution.