Join Date: Jan 2011
Location: Boston, MA, USA
Programs: FB LTPE, BAEC GGL, EK Blue, SK Gold, Marriott Amb+LTT, IHG Diamond Amb, Accorhotels Silver
Posts: 2,090
If the voucher is issued by Air France (057), it needs to be used on a ticket issued by Air France, that can indeed contain only DL flights.
If this is technically possible, it’s clearly not feasible online for a pure US domestic itinerary.
you can try by calling reservations but they might not want to do it either.