Does what still work?
You can add Additional Storage Locations from the DVR web UI now.
Click the + next to Storage Paths

It will then let you use a directory picker
Choose an additional storage directory for recordings
If your main DVR drive is filling up, you can move existing recording folders over to a new drive and add that as an additional storage path.
Note that new recordings will still be created on the primary DVR storage drive. Old recordings can be manually moved over, and will automatically play from the secondary drive if the video is not found on the main drive.