Start a ZFS RAIDZ zpool with two discs then add a third?

A method some people use, but that is not recommended:

1. Create a pool with 2 drives and a sparse file (of the right, virtual size to match the other drives), and then immediately offline the sparse file. This will create a degraded RAID-Z1 pool with two drives worth of capacity and no redundancy.
2. Copy data to the degraded pool.
3. Replace the offlined sparse-file with the third disk and resilver.

The number of "devices" that a vdev is created with, is permanent after creation. Therefore, you must have three "disks" at the time of creation. This method gives a way to create a degraded RAID-Z1 pool and later restore the absent redundancy.

This method is not recommended, because while copying and while the 3rd drive is absent, there is no redundancy/parity. But it can work.

Unfortunately, at the moment it is not possible:

It is not possible to add a disk as a column to a RAID-Z, RAID-Z2, or RAID-Z3 vdev. This feature depends on the block pointer rewrite functionality due to be added soon. One can however create a new RAID-Z vdev and add it to the zpool.

RAID-Z1 might work with just 2 drives, but clearly this is not very useful if extra drives cannot be added later. RAID-Z (like RAID 5) becomes more efficient, in terms of usable storage space, the more drives that are used.