Hello. The PERC 6/i and earlier including PERC 5/i do not support this operation. Only PERC H700 and above do support expanding storage space from hard drives. You may slice the array by creating another Virtual Disk (VD) from the available space. Note that sliced arrays are not be reconfigured. Otherwise, backup your data and replace the existing drives and re-create the array with new drives. Re-install operating system and restore data from backup.
I have an old PowerEdge 2950 with a PERC 5/i.  It has two sets of hard drives.   The mirrored set was running out of space, so I swapped one of the 30GB with a 100GB let it mirror then did the other.   It worked.   Went to expand the drive to use the new space, Windows doesn't recognize the new drive space.   Rebooted and did a ctrl R and went into the PERC utility, it does not recognize that the new drives.   It thinks the mirror is still 30 GB.    How do I get the PERC utility to recognize the new drives so I can expand to the new space without rebuilding the OS?
mirror hard drive