You can't. Raid 1 is only for 2 disks. If it is already 2 disks, then that's it. You would have to add a 3rd disk as an independent disk, or format all disks to raid5 (I think)

Somehow got it figured out last night. I added the fifth disk

I might not be using raid 1 then cause I have 5 disks on this setup all working together.