Option 3 of Microsoft KB 2009365 describes how to add a disk to Windows Server backup when other disks are offsite: you have to use the wbadmin command line tool. But the article does not explain how to add a volume, not a disk, as a target.
If you follow the KB article and answer Yes to all the command-line prompts, it will delete all data on the specified disk when it reformats it for exclusive backup use. Fortunately I stopped it before then.
To add a volume as a backup target, let’s say drive F:, use this syntax:
wbadmin enable backup –addTarget:F:
If the volume does not have a drive letter, you can use mountvol to get the volume ID, then use this syntax:
wbadmin enable backup -addTarget:\\?\Volume{e16904af-9171-11e2-9109-001d23bdb7ff}
Note that mountvol will include a closing backslash (“\”) in the volume name. Do not include that last “\” in the wbadmin command.
More info: TechNet’s wbadmin enable backup documentation.