Group demands across multiple part numbers

We have some cases where a vendor will require a minimum total order quantity (say 25,000 units) that can be spread across a larger group of individual part numbers.  We might order 5000 of item A, 10000 of item B, and 10000 of item C.  The planner will see initial demand for one of these, but it is not a high enough quantity to meet the vendor's minimum.  The planner must then look on the workbench for the next planned orders that meet the vendor's minimum.

Is there a way to group multiple part numbers together so MRP will drive their planned orders at the same time?  I'm envisioning something like fixed days supply that includes more than one part number.