MRP Planning for an item both manufactured AND purchased -

We have an item that we manufacture in several facilities.  In our primary manufacturing facility, it is a manufactured item but we recently are supplementing inventory by purchasing from another facility.

For us, this purchase order schedule is fixed and should not be rescheduled by MRP.  We need MRP to focus on planning the work order rather than expediting the existing purchase orders.

We continue to see MRP suggest to expedite the PO's and not suggest producing additional work orders.  Does anyone have any suggestions how we can accomplish this?  I do not believe you can freeze a purchase order like you can freeze a work order on the schedule.