You will have to use OMSA ... go to Storage, PERC, Virtual Disks, and choose Reconfigure from the dropdown menu of Available Tasks for your RAID 5.  This will add the disk to you RAID 5, then you will need to adjust the partitions in Windows once it is complete.
Hello all. I currently have a Dell PowerEdge R710 configured with 4 SAS-2 146G 10k Seagate 2.5" drives in a Perc h700 integrated RAID-5 array. I have ordered a SAS-2 146G 10k Seagate 2.5" drive to add storage to the RAID-5 array, making it a 5 disk array. What is the best way to add the drive to the RAID-5 array without losing any data? I have OpenManage Server Admin on the server. OS: Windows Server 2003 R2 32 BIT 
Add Disk Drive to PERC H700 Integrated RAID 5 Array