I have a Time Capsule that I'm using for my backups.

I have a network volume (coincidentally on the same time capsule) that I'd like to back up as well.

How can I tell Time Machine to back up network volumes in addition to my main laptop hard drive?

PS: yes, I know this setup isn't ideal. It'll incur 2x network overhead when backing up the network volume, plus my data won't be safe in the event of a drive failure since both copies will be on the same disk. However, it will give me some small amount of safety in the event I accidentally delete files on the network volume, among other things.