May I successively add a new hdd to an existing pool?

I would like to transfer all my stored data from ext3/4 to zfs. May I first build a pool consisting if two drives as raid-z1, then copying the data from a third disk onto it. As the next step wipe the third disk, add the third disk to the existing pool, then copying the data from the fourth disk to the 3-disk z1-pool. Going on the same way withall the other disks? All disks have the same size.
I would like to achieve a zfs pool with raid-z1 quality. In total there are 8 Disks, 6 filled...

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