Personalization - How to add Supplier Field (with LoV) to the 'View Cart and Checkout' Screen

I need to be able to add the Supplier field (with LoV) to the iProcurement 'View Cart and Checkout' screen.  The field in question is 'Supplier' from PoRequisitionLinesVO.  I can successfully add the field as message styled text, but need to be able to do it as a 'message lov input.'  I'm assuming if I knew what to put in the External LOV field that would help.  Any help or direction is appreciated.