The PERC 6/i does NOT support non-RAID, so the only way to add a single "standalone" drive is to configure it as a single-disk RAID 0. The PERC 6/i also does NOT support individual physical disk sizes above 2TB. If you upgraded your controller to an H700, you would still have the same RAID/non-RAID limitation, but you could use disks larger than 2TB.
I have a T610 with PERC 6/i SAS RAID Controller running 6 1TB disks in RAID5.  The client wants to add a standalone single 4 TB disk to offload data they are not yet willing to delete but is not in active use.  Is it even possible to add a standalone Drive that runs beside a PERC 6/i? 
Standalone Data Drive