Ah, RAID1 for the OS drive.
I don’t believe this constraint described here has been addressed yet:
though, I believe, if you use the btrfs command line, you might be able to handle it that way (you would still want to wipe the disk to add before embarking on that endeavor). However, from a practical standpoint, I would consider this probably overkill.
If you backup the Rockstor configuration via the WebUI and don’t store additional data on the Rockstor drive, a recovery of the OS takes usually a short period of time only. Of course, if this machine is mission critical to you (or others) you might think differently w.r.t. “allowable” downtime.