RAID group expansion is what you need - find it under RAID group management in the Web GUI
Check the Web GUI manual for further details but the biggest thing to note is to backup all your data on the RAID group before expansion. RAID expansion can take a LONG time, especially if you are adding 5 disks (I think it will let you add 5 disks at once but logically it adds them one at a time). So this expansion could potentially take days, if not weeks, and the data could be at risk during that time.
Also (from the manual) When expanding drives, use drives that have the same capacity and
speed as the drives that configure the target RAID group. If drives of
different capacities exist in a RAID group, the smallest capacity
becomes the standard for the RAID group after expansion, and all other
drives are regarded as having the same capacity as the smallest drive.
In this case, the remaining drive space will NOT be used. In addition, if
drives of different speeds exist in a RAID group, the access performance
of the RAID group is reduced by the slower drives.