Q: i am setting u a new time capsule. i already have a wifi setup in my office. now i want to use this time capsule just as a wireless backup machine. do i still need to attached a DSL cable, or it can connect to my existing wifi and start working ?

Please see this post for more information:

https://discussions.apple.com/message/18254250#18254250

You probably can, but this type of configuration is not recommended or supported by Apple.

Two cautions if you want to try this:

1)  The Time Capsule has to be configured to "Join" your existing wireless network in this type of setup...and to do that, you must know the exact type of wireless security that your network is using.

2)  Backups are likely going to take twice as long. The wireless signal must travel from your computer to the wireless router and then from the wireless router back to the Time Capsule.  Two "hops"......even if the computer is located close to the Time Capsule.

If you want to try this, we can tell you how....but with no guarantees on your results....since this is not officially supported.