Yes. You can format any disk at any time to any filesystem simply by stopping the array and going to that drive's page and change the filesystem. Then when you Start the drive will be formatted to the new filesystem. Formatting a drive is simply writing an empty filesystem to the disk, which is treated just like any other write as far as parity is concerned. And you can mix filesystems as much as you want as long it is one of the supported 3, ReiserFS, XFS, btrfs. (And of course parity doesn't have a filesystem.)