Comment by uniqueuid
Yes but see my sibling comment.
When you expand your array, your existing data will not be stored any more efficiently.
To get the new parity/data ratios, you would have to force copies of the data and delete the old, inefficient versions, e.g. with something like this [1]
My personal take is that it's a much better idea to buy individual complete raid-z configurations and add new ones / replace old ones (disk by disk!) as you go.
I wish something like this would be build into ZFS, so snapshots and current access would not be broken.