You might be able to configure the AirPort Extreme to "Join a wireless network".......IF.......you have an Mac running an older 5.x version of AirPort Utility. That would be a Mac running either Leopard (10.5.x) or Snow Leopard (10.6.x). A PC with AirPort Utility for Windows installed on the device might work as well.
The "trick" is that you must first go ahead and set up the AirPort Extreme to "Extend a wireless network", and then go back in using AirPort Utility 5.x and change the setting for Wireless Mode to "Join a wireless network". To get the "Join" option to appear, you have to hold down the option key on your Mac or PC while you are clicking on the selection box.
This works just fine when I test using a 4th Gen AirPort Extreme. I doubt that it would work on a newer "tower" shaped AirPort Extreme 802.11ac though since this device uses completely different firmware.

Unfortunately, the USB port on an AirPort Express will only work with a supported USB printer, so while it would be physically possible to connect a hard drive, it won't work if you try to find the hard drive on the network.