# fdisk /dev/sdb

# cryptsetup luksOpen /dev/sdb1 new_disk

# mkfs.ext4 /dev/mapper/new_disk
# mkdir /newdsk

Use the following to get the UUID for the partition/disk.
# blkid -s UUID -o value /dev/sdb1
# vi /etc/crypttab
…
new_disk UUID={result from blkid command above} none luks
…

# vi /etc/fstab
…
/dev/mapper/new_disk /newdsk ext4 defaults 0 2
…

# systemctl daemon-reload
# mount /newdsk

When you add a disk to a system that already has an encrypted disk, say the system disk, you will prompted twice for the passphrase. Once for each disk.