Can I transition to a RAID 1 setup from a single drive?

No, you can not create a RAID 1 setup from a single drive without losing the data on the original drive(s). You may want to consider leaving it in JBOD mode and using software

to sync the drives.

Another solution is to move the data to another drive, set the RAID 1 on the Mobius unit, and then move it back.