How to default S.loc address in SRM po instead of plant address

Hi,

     I need to default the storage location address in purchase orders instead of plant address in extended classic scenario (SRM 7.0).its not for all the plant but i have plant with two builidings i want if Shopping is created without entering s.loc address it should pick plant address but if S.loc address exist in SC then it should pick the Storage location address instead of Plant address. during purchase order creation and print out.

     Can someone provide the steps that i need to perform in system.

     Thanks