I believe you can assign it to a school, but I don't think it is recommended to do so until you receive the letter. A few people have had the misfortune of assigning an unreceived letter to schools only to have that letter writer flake and they have to notify each school that the letter they assigned will not be coming. From what I understand, you have nothing to gain by assigning an unreceived letter and can potentially make more work for yourself if that letter writer bails.