I'm afraid you won't be able to use the free space in any supported way. With the extents you are limited anyway and creating an additional datastore on a LUN is not supported by VMware (1 LUN -> 1 Datastore).
If you have free disk space on the storage system, I'd suggest you consider to create new LUNs (max. 2TB minus 512 Bytes) and migrate the VM's over to these LUNs to get rid of the extended datstore.
André