I had a zpool with 4x8TB SATA drives in a RAIDz2 with a total capacity of 16TB. I updates to PVE 9 and ZFS to 2.3 and attached 2 additional drives to the zpool using:
zpool upgrade <poolname>
zpool attach <poolname> raidz2-0 /dev/disk/by-id/<drive1>
zpool attach <poolname> raidz2-0...
