Not going to possible, the volume can be extended on the fly but not shrunk. There is no diskpart command to shrink a volume.

You could either get a new LUN attached and mounted under the root drive and then move the files to the new drive or you could have the current LUN mirrored to a new LUN which you then attach to the server and then move the db files. You'll need to stop and restart the instance so do this within a maintenance window.