You can always create a 2nd volume using additional drives. But I'm not sure about adding them to existing RAID 5 array. I might be wrong, but as far as I know this feature is called "Online Capacity Expansion" (OCE) which is only available on Intel hardware RAID controllers.
You may also check with the experts in the Graphics and Chipsets forum.
Online Capacity Expansion is supported by the Intel Matrix Storage Manager starting in version 8.5. It is also supported in all versions of the Rapid Storage Technology product (which is a follow on to Intel Matrix Storage Manager). After you add the new disks (which must be at least as big as the smallest disk currently in the array), you will see the "Capacity Expansion" option when you modify the volume.
Your answer came after the fact ;-)
After 20 hours of no reply I decided to go ahead and just try it. I added the two new drives to the array and started the Intel Matrix Storage Manager 8.9. The drives showed upped. The right-click option was to add them as a "Hot Spare". Right-clicking the volume name gave me an option to "Modify the volume". You get a wizard which allows you to "Add one or more member hard drives". This was starting to look shiny. I added the new volumes to the array and got the finish option. Now what I did not know, was, what would happen after I finish the wizard. I could re-initialize the array for all I know and loose all my precious data. 2.5 TB of it.
Have you ever tried a mouse-click that could loose you 2.5 Tb? <grin> I had not and I trust Intel so I clicked Finish. The volume expanded to roughly 4.7 TB and a migration process started automaticly. It worked great, I had access to all my existing data while migrating and after 20 hours or so the migration process was done. I now had successfully added two new drives to the array.
Started the Windows 2008 partion manager which let me extend my excisting dynamic volume with the new found space on the array. Operation was a total success.