Use the simplest solution to this problem.
Don''t try to adapt other tools to do this (scheduling agreements is not exactly for this) or use Z routines to do this automatically because it is not necessary just to create the purchase requisitions.
If you are obliged by contract to buy material XY every month, just create manually a purchase requisition with several lines, each one for one month.
This purchase requisition is fixed (because you have created it manually) and MRP will not delete it.
If your demand is bigger than the MRP will create another purchase requisition, but you must inform the material planner to avoid this purchase requisition deletion, because sometime you could not have enough demand for the material and MD04 will suggest you to delete the purchase requisition or change the dates.
Regards,
Daniel de Assis Vieira
SAP PP/MM Certified Consultant