MRP Managed materials supplied by another plant

Hi Gurus,

I have two plants. One plant is a main distribution plant and supplies inventory to other plants. In one plant (Store) they procure some inventory from local vendors and some inventory is supplied by the main store (Plant).

I must set up MRP to manage all materials which is no problem, however, when MRP generates a supply document it cannot differentiate between the Main Store and the Local Vendor.

Either all documents will be Purchase Requisitions - for the local Vendor or they will be Planned Orders to be converted to STO's, but then this step is manual.

How can I set this up in the best way to reduce manual steps?

Thank you kindly