RE: Adding RAID 1 on a Poweredge T320 without reinstalling OS.
So, be careful how you define "no RAID", because the difference is the answer to whether you can or can't.
Some controllers do NOT support non-RAID, so a single disk would be configured in a RAID 0. If this is the case, then YES, you can convert it to a two-disk RAID 1.
Some controllers support non-RAID, so a single disk could EITHER be a single-disk RAID 0 or a non-RAID volume ... if it is a non-RAID volume, then NO, you cannot convert it to RAID, as RAID and non-RAID are not compatible, and you would be better off mirroring in the OS. (Or you could do a backup/restore to RAID.)