Airport Time Capsule is the most effective Wi‑Fi base station backup solution for Macs. It is designed to support working with Time Machine in Mac OS X back up your Mac automatically and wirelessly. The key is How to hook up a Airport Time Capsule to Airport Extreme for larger range of backup?
Go to Airport Utility > On the Internet tab choose dhcp.. nothing else to fill in.
On Wireless tab, type wireless name.. you must create a wireless network.. if you want just one wireless network then use same name and same security as the Extreme already has set.
In another case, if you want to hookup upgrade from airport express to airport extreme, allow the Airport setup wizard to replace an existing apple router. So click on options and select “replace an existing device” instead of “Add to an existing network”:
Just a brief tutorial for hooking up a Airport Time Capsule to Airport Extreme for larger use of backup. Hopefully it would help.