Can I put an Airport Extreme in client mode (sharing printer only)?

I have an Airport Extreme 3rd gen (A1301). I just bought a Time Capsule 5th gen (A1470). I put the TC where the AE used to be, in a cupboard in the hall. Now I am thinking to repurpose the AE as a printer server on my desk.

Can I configure it so it connects to the wifi of the Time Capsule in client mode? I don't want to extend the network of the TC since then I would risk connecting to the Airport Express instead of the TC with my other devices, which would be undesirable.

On a Windows version of Airport Utility ctrl-click on the "wireless mode" menu will give you an additional option "join a wireless network". Without ctrl the option is hidden.

You can do the setup to extend the network and then change it to 'join a wireless network' afterwards under the Wireless tab in airport utility. That's how I setup mine

Yes, you can configure an Airport Express to connect to an existing WiFi network in client mode.

From Apple's support site, AirPort Express: How to join an existing Wi-Fi network in client mode:

1. Connect an appropriate cable between the AirPort Express and the device it will serve.
2. Plug the AirPort Express into a power outlet.
3. After the AirPort Express starts up, select it in the AirPort Utility screen.
4. If you are now repurposing the AirPort Express, first reset it from Base Station in the menu bar and select Restore Default Settings.
5. After the AirPort Express completes the restore process, choose it from the Wi-Fi menu in the upper-left corner.
6. Click the Other Options button.
7. Select "Add to an existing network".
8. Choose the Wi-Fi network name from the Wireless Network Name menu that you wish to join and click next.
9. Enter the password to this network if required.

The question was whether one could configure an Airport EXTREME in client mode. The first "answer" answered whether an Airport EXPRESS could be configured in client mode.

I am using Airport Utility 6.3.5 to configure both an Airport Express and an Airport Extreme. When configuring the Express, "join a network" is an option; but not with an Extreme. However, when I configure the Express to "create network", and then configure the Extreme to extend that network, the Extreme does in fact forward packets received on its LAN ports wirelessly to the Express, and onward towards the internet. So the defacto result is that the Extreme is acting as a "gaming adapter", or "client bridge".

Note: after I configured the Extreme to extend a network, I went back to see if the "join" option became available; it had not.