Actually no, this is an AEBS and not a Time Capsule. Backing up to an external drive on an AEBS is not supported by Apple (not that it can't be done, but it's tricky to set up and unreliable).
My suggestion is (1) plug the external drive into the iMac, use TC for that and (2) get Mountain Lion Server ($19.99 from the App Store) which will let you also back up your Macbooks to the iMac.
There is no need to partition the drive; you can back up multiple machines to the same volume - the only reason you might want to partition is if you want to dedicate a certain amount of space to some computers versus others (e.g. limit the space that the kids' computers can take up on TM backups while giving your computers more room for TM historic backups)