You can move mailboxes of several domains to new volume. For example:
1: Mount new volume under /mnt/newvolume/
2: Stop Postfix / Dovecot / Nginx services, make sure no process will update mailboxes during change.
3: Move /var/vmail/vmail1/<domain1> to /mnt/newvolume/, then create /var/vmail/vmail1/<domain1> as a symbol link to /mnt/newvolume/<domain1>/. For example:
mv /var/vmail/vmail1/domain1.com /mnt/newvolume/
ln -sf /mnt/newvolume/domain1.com /var/vmail/vmail1/domain1.com
4: Repeat step 3 if you need to move more domains to new volume.
5: Start postfix / dovecot / nginx services.