In OMSA, go to Storage, PERC, Virtual Disks, and choose Reconfigure from the dropdown menu of Available Tasks for your RAID 5.  It will start a wizard, walking you through adding the new disks to your RAID 5.  The physical disks must be showing as READY first (if showing Foreign, that must be cleared before you can do anything - Storage, PERC, Information/Configuration, Foreign/Clear from the dropdown for the controller).
I have: Poweredge R610 PERC 6i 3 x 146GB SAS in Raid 5 OS: Hyper-v Server 2008 r2 I just added three more 146GB SAS drives How do I expand the RAID 5 from here?  I've been reading up on Open Manage but have not seen a clear path to get this working the best way.  Thanks in advance.
Poweredge R610 add hard drives to Hyper-V Server 2008 r2 PERC6i