Is it posible to overprint a fountain color on a spot color in Coreldraw?

Dear all,
it's time to deal with a problem on which Coreldraw is not helpful. I have made a fountain from 50% to 0% black that I want to overprint on a spot color. Whatever I do, when I save the file as pdf or eps, the fountain does not keep the overprint feature. My atelier can not create the separations and the ask me to send new correct files. Is there anything I can do?