actually, you can add additional storage to a time capsule. im not exactly sure if it shows up as a separate drive. (i would assume so though)
but if your looking to backup the attached drive, no,i dont think that time capsule cant do that, at least not through time machine.
ive always wondered, what happens if time machine is unable to do a backup every hour? lets say that i am traveling and my backup drive isnt plugged in. do i get an error message, or does time machine automatically backup the next time osx senses the drive?