I was thinking exactly the same as BongoHunter but decided not to say it at the time as it looks like this may be an upgrade. Adding drives to an existing ZFS pool currently involves adding a whole new vdev (last I heard a solution for that was being worked on, but filesystem development is typically very slow, for good reason). Unraid, on the other hand, can take new drives any time, so long as the parity drive(s) are always the largest in the pool. You do sacrifice performance for that flexibility, so both have their merits.