Dear Stephanie why don't you use scheduling agreement,
"Delivery of the total quantity of material specified in a scheduling agreement item is spread over a certain period in a delivery schedule, consisting of lines indicating the individual quantities with their corresponding planned delivery dates."
More can be found here: Scheduling Agreement - Purchasing (MM-PUR) - SAP Library
However if you want to stay with PO, I recommend you use user exits to control the process, here you ca find bunch of information regarding MIGO user exits: User EXIT for MIGO
Cheers,
Sarhan.