You can expand the pool not only by another RAIDZ-1 (in your case), but with any kind of ZFS volume.
My ultimate goal is to have 2 pools of 6-7 disks (in RAIDZ1 ideally). If I add a disk in another format than RAIDZ1, as a basic disk or a mirror, yes, it will be present, but not in RAIDZ1, is that it?
If my request above is correct, you must at least add at least 3 disks for RAIDZ1.
Regarding SnapRaid I will look at this thank you.
Edit: Following the reading on the forum topic about snapraid and a very good comparative table, I think indeed that snapraid is the most flexible for the use I want to make. I was going to explore in this way. thanks again @cabrio_leo