Can I back up other external drives to my Time Capsule using Time Machine??

My external HD is a Seagate FreeAgent GoFlex Desk 2TB drive formated with Mac OS (Journaled). It's networked into my 3 TB Time Capsule through the USB port on the Time Capsule. I'm using two MacBook Pros, both running Mac OS X Lion.

I want to use the Seagate drive as a primary storage for documents so they are easily accessible from both MacBook Pros. Since that's where most of our data will be stored, I want to back it up. Using Time Machine to do that seems like the easiest option, if it can be so configured.

Any advice?