Yes you can! With the AirPort Extreme you can connect a harddrive and use it as an "AirDisk" or as a Time Machine back-up destination. If you want to use 1 drive to do both, however, this can cause problems. You could partition the drive in 2, but if you ever needed to change partition sizes, you would loose all of your data.
What I would do is buy two hard drives, and use one for TM backups and the other for network file storage. I do something similar where each Mac has it's own Time Machine back up drive connected to the Time Capsule.
Hope this helps.
Dylan.