Step by Step tutorial on How to Add Airport Time Capsule to Existing Network
Step 1: Open Launch Pad and select Airport Utility.
Step 2: Click other Wi-Fi Devices and Select Airport Time Capsule.
Step 3: Click Other Options
Step 4: Select Add to an existing network and select next button.
Step 5: Select your existing network then press next
Step 6: Name your Time Capsule and create a password
After this steps, just wait for the setup to finish.
Watch this for video instruction: