Below steps which needs to be taken to extend system (or any other drive for that matter) on Windows 2008. No reboot is required for this method.
Initial configuration is below
1. Add additional drive. No reboot is required if it’s virtual OS. Initialize the drive with MBR partition.
4. Delete volume which is left from old drive on original disk.
5. Choose extend volume from OS drive. This will extend volume with no reboot.