How to share the Internet connection with another MacBook via WLAN

Yes you can. Since the Time Capsule is an Airport Extreme with an internal hard drive it's relatively easy to convert it into a wireless to wired bridge assuming you already have it set to hook up to the wireless network.

1. Open up Airport Utility and start Manual Setup on your Time Capsule
2. Go to the Internet section
3. Under Internet Connection be sure Connection Sharing is set to Bridge Mode

If bridge mode is turned on it will share the connection and essentially act like a switch.

The answer you're looking for, though, is Internet Sharing:

1. Go into the System Preferences on your iMac
2. Go to Sharing, then click on Internet Sharing
3. In the dropdown, indicate that you want to "Share your connection from" Ethernet
4. Check the Airport box
5. You have the option to set your network name, channel, and to use WEP encryption by clicking "Airport Options".
6. Finally, check the box next to "Internet Sharing" on the left hand side.
7. At the prompt, click Start.

You should now be able to connect to the internet on your iPad by selecting the network you set up on your iMac.

USB sharing is also now easy and fast: Can I share my iMac's internet to iPhone or iPad over USB?