Call DL. Ask to be transfered to the AF/KL support desk as you need to book a domestic DL flight but are using an AF voucher for payment.

Agent there can go into AF system to make booking and apply voucher.. (Note this only works for vouchers, not for unused eticket rebookings)

AF should also be able to do this directly. (The DL agent above would be selling ticket using the AF system, just as if they were an AF agent) AF agent just needs to know how to sell a DL fare. There's of course no AF fares for domestic US only reservations. This is why AF agent says no if don't know what to do. (AF can sell any DL fare just as the could sell any KLM fare. They just normally of course default system to only their own fares)