That's not what I'm looking for. I'm not using File Vault right now. I just want to back up my main drive into an encrypted disk image on another drive. I'm not really worried that someone is going to steal my whole computer, but someone could take my external drive and look through it or copy it pretty quickly, or just walk away with it.
I just want to know if I can use an encrypted disk image as the volume that Time Machine backs up into.