That you will, when the PR is considered fully stable and good to go, be able to add drives to a RAIDZ vdev.
Here we are - https://arstechnica.com/gadgets/2021/06/raidz-expansion-code...
Here we are - https://arstechnica.com/gadgets/2021/06/raidz-expansion-code...
I ended up freeing up that SATA port I needed, but I'm waiting just a bit longer to expand ZFS on to that last drive so this code can get juuusstt a bit more testing!