You cannot reconfigure a "sliced" disk group.  You said you have "2 Virtual Disks" ... if this is true, and you have created two RAID 5 arrays across the same set of disks (called slicing), then you cannot reconfigure.  You would have to do a backup restore and delete at least one of the RAID 5's. If, by "virtual disks", you mean Windows partitions, and you really only have one RAID 5 (Virtual Disk) across the disks, then, yes, this can be done as Chris described.
I have a Poweredge T410 with the H700 controller.  I have two 250GB drives configured as Raid 1 and three 2 TB drives configured as Raid 5.  The Raid 5 is partitioned into 2 Virtual disks of 1.8 TB.  I need additional space so I want to add a drive to the existing Raid 5 configuration.  I installed an additional 2 TB drive and the drive is recognized but I only have the option to add it as a hot spare.  How can I add it to the Raid configuration so I can create an additional Virtual Disk.
Adding drive to T410 Raid 5 