This is will be still RAID0. Please, ask expert @SGC if you do not believe me.
ZFS doesn’t support adding disks to existing RAIDZ pools with parity re-configuring. So, even this beta feature is technically available for ZFS, it’s useless - the speed will not increase, but risk of something would go wrong is increasing.
You can do only a mirror. This is not possible for disks of different sizes.
The ZFS has a lot more restrictions than LVM for example.
The mirror doesn’t help to extend storage.
So, adding only one disk is not possible. You need to convert the whole setup to something like RAID5/RAID6/RAIDZ/RAIDZ2. And this is requires more than one disk.
And I do not recommend to use RAID5/RAID6 without checksums and auto correction anyway. So the only ZFS is viable at the moment.
Do not offer BTRFS. It’s alpha and not ready for production independently what they are saying. Their RAID implementation is nightmare and the route to data loss.
For the node the RAID is not needed, the network itself has a redundancy, this additional local redundancy will not be paid, thus - waste of resources and wearing disks much quickly than with a 1:1 node:HDD setup.
Please see also