Wait, I thought you need 7? Having a ADR for 6 isn’t going to help get you an extra person seated with you. Your adr for 6 is seated at a table for 6. They won‘t just pull an extra chair up. They won’t pull another table up to let you add an extra. I would keep the time you want & keep trying for an adr for 2 near the same time. Then ask if you can be seated together. They will try to move 2 booked tables together. They won’t pull another table you don’t have booked up to let you add an extra.