MRP run

Hi,

Our demand is from Sales order. After the MRP run, the Po will be issue for this product. After we get the product from the vendor we will do a good receipt, when the GR done the qty of the product will increase. But in our system, we don’t keep the product qty in SAP but in another system. So, once GR is done, it will be issue out from SAP. We are having problem here, if we issue out the product and always keep the inventory in other system, then MRP run, it will always raise an PR to create the PO because the particular product inventory is always 0. So, I was wondering if there is any field for us to input inventory qty, so that MRP can run correctly based on the qty input? Or maybe there is other work around?