I think you’re thinking of a vdev, and you’d need at least three disks to do that.
You can simply add the two disks, one at a time, to your vdev, following the guide Samuel linked, and then replace the other three, leaving you with a five-wide RAIDZ1 vdev. However, RAIDZ expansion keeps the existing data:parity ratio for any existing data, and often results in available space being reported incorrectly. You can address this, at least to a degree, using zfs rewrite, as described in more detail here:
