ZFS does not allow adding a new disk to an existing vdev. https://www.truenas.com/community/threads/add-a-drive-to-existing-vdev.76511/
https://www.delphix.com/blog/delphix-engineering/zfs-raidz-stripe-width-or-how-i-learned-stop-worrying-and-love-raidz https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-not-raidz/ https://pthree.org/2012/12/13/zfs-administration-part-viii-zpool-best-practices-and-caveats/