If you're willing to use mirror vdevs, expansions can be done two drives at a time.Also, depending on how often your data changes, you should check out snapraid. Doesn't have all the features of ZFS but its perfect for stuff that rarely changes (media or, in your case, archiving).

Also unionfs or similar can let you merge zfs and snapraid into one unified filesystem so you can place important data in zfs and unchanging archive data in snapraid.