No, that is not possible. RAID-Z vdevs cannot be expanded by adding more disks. The only way to expand them is to replace the disks with larger ones, one at a time, letting the system rebuild the vdevafter each replacement.
The story was misleading. It read:
Similarly with ZFS, when you need more hard drive space, you put in another hard drive and you’re done. No need to spend time partitioning, formatting, initializing, or doing anything else to your disks – when you need a bigger storage “pool,” just add disks.
If you are creating a zpool from single-disk vdevs (as in the first example given), then this will work. But there will be no redundancy as the zpool will stripe over each disk with no parity. Another way this might work is that if you already have a mirrored vdev, you can add another disk to that vdev, but this expands redundancy (i.e. creates a triple-mirror), not capacity. While the statement in the story is technically correct, since most people associate ZFS with its RAID-Z functions, it is quite misleading to the casual reader. Adding disks in this way to a pool is a recipe for data loss.
It is important to understand the difference between zpool and vdev. The vdev is the virtual device that is made up from block devices (e.g. hard drives). A vdev can be a single disk, a mirror or a RAID-Z. A zpool is a collection of one or more vdevs into a single storage device accessible to the file system. A zpool always stripes data across vdevs. This is an important detail, because it can lead to trouble if not considered carefully. For instance, if you have one RAID-Z vdev in your zpool and you want to expand your storage with another set of drives in a new RAID-Z vdev, you can create it and include it within the original zpool. And while each of those vdevs has the ability to recover from a single drive failure (or more in the case of RAID-Z2 and RAID-Z3), two drives failing within the same vdev will cause the entire zpool to be lost, even though the second vdev had no drive failures at all.
So it is best to carefully plan ahead for any ZFS deployment as a filesystem this sophisticated naturally has a learning curve. Having said that, I think that ZFS is the only way to go for a DIY NAS. I use FreeBSD for mine, but for beginners, I might recommend FreeNAS.