OMSA, Storage, Virtual Disks, Reconfigure from the dropdown menu of Available Tasks for the RAID 6.  It will start a wizard where you will select the disks to add and the RAID level to convert to (leave as RAID 6).  It will take some time, depending on the type of drives, the size, and the usage.  There will be some degraded performance during this process.
We have a PowerEdge R520 with 5 drives installed.  It is using the PERC H710 Mini controller.  The drives are in a RAID 6 configuration.  We are adding 3 more drives (same size).  We currently have OpenManage Server Administrator 7.2.0 installed on the machine. What is the procedure to bring the new drives into the array? Thanks
Add Drives to existing RAID 6