Access to SO Drop Down on Non-Drop Ship PO"S

We are a custom equipment mfg, and nearly all PO's are purchased against an existing Sales Order. We use a UDF to designate the SO on the PO. It would be nice to use the existing SO drop down and data field that exists in the PO detail table. Is there a way to use this feature on PO's that are not drop ships? 

Thanks all.