Microsoft Knowledge Base Article 304736 contains the following summary:

This article describes how to add additional storage capacity to a server cluster if the underlying hardware RAID supports "capacity extension" technology. Capacity extension provides the ability to add additional drives to an existing RAID set and extend the logical drive so that it appears as free space at the end of the same logical drive. You can use the Diskpart.exe command-line utility to extend an existing partition into free space. This process has the following requirements:

The additional disk space must appear as free space at the end of the existing drive, and it must be directly behind the existing volume that is to be extended.
The extension must not rely on software fault tolerance to combine the existing partition and free space.
The disk signatures of the existing drive remain the same.
Use of the Physical Disk Resource type for the disk. If the disk resource is provided by a third-party manufacturer, you must contact that manufacturer for information about how to increase disk space.