You will need to backup the data on your disk, add the second drive, configure RAID 1 in the controller BIOS for the SAS 6/iR (CTRL-C), then restore your data.  The SAS 6/iR supports RAID and non-RAID, but they are not interchangeable; when you configure RAID, special metadata for the array specifications will be written to the disk, overwriting anything currently there.  The array will then be initialized, which will wipe out any remaining data.  Backup/restore is what you want to do to build a hardware RAID.  If, instead, your main goal is to NOT have to backup/restore, then you will need to mirror the drive within the OS.
I have two Dell Seagate 300GB 15k SAS drives on a 6/ir controller in a Dell T610 tower server.  The first drive has been used outside of a RAID array and is nearly full with valuable data. I just bought a new drive with the same specs to add to a not-yet-existing RAID 1 array. I would like to build the array without losing the original data on the first drive. Is this possible?  Thanks for the help!
Creating RAID 1 array on 2 drives w/o losing data on the 1st drive 