First I would backup and verify the backup
Then could add the two drives and if they are larger you could create a mirrored NSS partition, then use VCU to migrate vol1 to the NSS volume.
After you verify your data is safe on the new volume you could delete the old volume and partition, create a new NSS partition, mirror it, assign the new partition to your new nss pool, and expand vol1 across the whole 4 disk set. Unless you are running an ap like ACT that doesn't work well on NSS, NSS volumes are much preferred to Traditional.