Sorry ... I see you included the controller in your title - probably helpful to also include it in the body.  Anyway, the SAS 6/iR cannot convert your single non-RAID disk to a member of a RAID 1.  You can backup/setup RAID 1/restore, or you can set up a RAID 1 managed by the OS, if you really can't/don't want to backup/restore.
Hello,   I have a Dell T300 Server with 3 hard drives.  The first two are configured as a RAID-1 array and contain the OS on one partition, and data on another.  The third drive is for data and also contains a database which I do not want to risk imaging or losing. I would like to add an additional drive, identical to the third, and set it up as a mirror for data redundancy.  I have sourced an identical drive.   Is it possible to add the second drive and simply "convert" to a RAID-1 array without losing the data on the original drive?
T300 SAS/6iR, convert single hdd to two drive RAID-1