RAID 5 requires a minimum of 3 drives. You need only configure a new VD - be sure to check the Advanced and Initialize boxes during creation. Insert the drives HOT (do not power down to insert them), then reboot to the CTRL-R utility.
I have a Dell R510 with 4 Drives configured with RAID 5 being used with vSphere 5.5 - ESXi I have 2 open slots in the R510 server. Is it possible to insert two new drives and initialize them to make storage available  to ESXi 5.5 without disturbing the RAID 5 configuration currently defined.  thanks
Adding Drives to RAID Config ?