What ZFS are you using?
“As of November 2023, this feature was merged in main and is scheduled for release in OpenZFS 2.3:”
'The OpenZFS project (ZFS on Linux, ZFS on FreeBSD) is working on a feature to allow the addition of new physical devices to existing RAID-Z vdevs. This will allow, for instance, the expansion of a 6-drive RAID-Z2 vdev into a 7-drive RAID-Z2 vdev. This will happen while the filesystem is online, and will be repeatable once the expansion is complete (e.g., 7-drive vdev → 8-drive vdev)."
Not sure on MergerFS…I haven’t gotten deep into ZFS pools.