I can verify from multiple attempts, with multiple brands, that connecting an all-in-one to the USB port on an AirPort Express or AirPort Extreme will allow printing but not scanning.
itickings is correct, ravenvii is (sorry,) wrong.
(ravenvii) You may have been using an all-in-one that has its own built-in networking interface, and had it connected over the NETWORK connection instead of the USB connection. I have successfully done that multiple times (and presently have a network-enabled HP all-in-one connected that way.) But, again, it will not work over the USB port.
AirPort base stations only support two functions over their USB port: printing (note, printing, not printers, specifically means that it only supports printing, no matter what functions the printer may have,) and, in the case of the the 802.11-n AirPort Express, it also supports hard drive sharing.
So, no, your all-in-one may work for printing, but it won't work for scanning plugged into the USB port of an AirPort Express. And if it has memory card slots, those won't work, either.
However, if you have an all-in-one that has a network port, you can use the network connection to connect to your computer wirelessly via AirPort. (The cx7400 does not have networking.) Some printers and all-in-ones have built-in Ethernet, and some even have built-in Wi-Fi. And, they're getting cheaper. I recently saw an all-in-one by Brother with Ethernet for $99, and one with Wi-Fi for $149.