Online volume extension is definitely supported for the scenario you describe, provided the increased size of a SAN attached drive is added so that the original volume as seen by the servers gets additional free space starting at the end of the existing partition. This KB article is for older versions of Windows Server but it details the procedure to follow for either online or offline expansion. This refers to using DiskPart not the GUI Disk Manager\Storage Manager but this should work with Windows 2008 ( as per this MSDN Blog entry ). If you are not getting the option then something else appears to be wrong, are you certain that the additional space is actually contiguous? The only other thing I can think of is that the volume size is too large to extend (2TB already and you are trying to make the shared volume bigger).
You shouldn't be using Dynamic Disks with failover clusters without third party software, that is why the Dynamic Disk option is disabled, the blog link above discusses this too.