What you are basically saying is that you buy (as an example) 1 container from your supplier every quarter. You of course want to fill up this container fully to get the best deal and save on freight, etc.
Now MRP will generate PRs automatically when there is enough demand. When MRP computes quantities it needs to procure to keep your replenishment fulfilled, it still has not performed sourcing. Sourcing is performed for PRs created - its a second stage. So obviously MRP in SAP ECC does not aggregate by supplier.
At this stage your planner, based on human intelligence, adds to the PO to fill up the container. QS: How do you expect SAP to chose which additional products to buy? There is obviously no demand, else PRs would already be created by MRP and be converted to PO?
1. You could try increasing planning horizon to force SAP to plan ahead and generate more PRs.
2. The other way to force MRP to 'create more PRs' is to add demand manually - planned independent requirements and re-run MRP to create PRs. However this does not reduce work - you may as well add materials to one PR or a PO.
A custom solution/utility is of course possible depending on specific needs.
Its worth checking in APO-SNP suite offers such capabilities. Or other such new dimension products from SAP.
I am sure the forum would benefit from whatever solution you decide on so please update this post