So that I am clear on things, your Time Capsule is now providing a wireless network and you want the AirPort Express to provide additonal wireless coverage for the same wireless network as the Time Capsule. Is that correct?
Or, is it your goal to set up a new wireless network with a separate log on for the area at the back of the house?
Sure, you can check your settings for the AirPort Express using AirPort Utility - Manual Setup
Click the Base Station tab below the icons to make sure that you have named the device, established a device password, adjusted Time Zone settings, etc.
Click the Wireless tab next to the Base Station tab. Settings should look like this:
Wireless Mode = Create a wireless network
Wireless Network Name = Same name as your Time Capsule wireless network
Check mark not needed next to "Allow this network to be extended"
Radio Mode = 802.11 n (b/g compatible) a good choice for most applications
Channel = Automatic
Wireless Security = Same setting as your TC wireless network
Wireless Password = Same password as your TC wireless
Click the Internet icon, then click the Internet Connection tab
Connect Using = Ethernet
Connection Sharing = Off (Bridge Mode)
Update to save any changes
It is important to power down and restart the entire network so that everything will be in the right sequence. Power off everything in any order you wish. Wait a moment, then start the modem first and let it run for a moment by itself. Then your TC the same way, then the Express, then devices, etc.
You should be able to "walk" your laptop from the area near the TC to the area near the AirPort Express and it will automatically switch to the access point with the strongest signal.