We have a similar environment (MS Windows 2008 + SQL Server 2008 MSCS). I often have to expand the LUNs to accommodate database growth.
Just grow the volume first to cover the size you want to expand the LUN by. Then, from within SnapDrive on the Windows node that his hosting your SQL Application (and owns the disk you want to expand) expand the disk to the size you want. We run this during full production hours with no downtime whatsoever.
Just be aware once you grow an FC LUN, you can't shrink it.
Check the SnapDrive admin guide for resizing a disk: