Buy some clamp on downtube shifters and appropriate cables w/ end fittings that mate w/ the shifters, and install after removing the other shifters of course. Retain or replace the short bit of cable housing at the rear derailleur. Ride.
Are your stem shifters indexed or friction?
__________________
1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"