The easiest way to do this is to use mirror vdevs, that way you can add disks two at a time. Of course, you miss out on RAIDZ functionality that way, but that's the simplest setup.
I've gone ahead and actually created a new array using btrfs raid10 instead (with an eye towards a RAIDZ2/3 like configuration whenever the btrfs guys get around to fixing it); btrfs allows for online reshaping of the mirroring configuration, so it doesn't have the limitations of ZFS in this instance.
I've gone ahead and actually created a new array using btrfs raid10 instead (with an eye towards a RAIDZ2/3 like configuration whenever the btrfs guys get around to fixing it); btrfs allows for online reshaping of the mirroring configuration, so it doesn't have the limitations of ZFS in this instance.