Using the Armory client, is it possible to specify one of my existing "receive" addresses in my wallet as a "change" address when I create a spend transactions? (Instead of the Armory client creating a new "change" address)
Yes you can, but you have to switch to "Expert" usermode from the main window. After you do, the "Send Bitcoins" window will have an option for customizing the change address. You can click on the addressbook and select any address in any wallet. Or really put in any address you want.
Just remember that you are compromising your privacy by reusing addresses. Even when you don't reuse addresses, there's a surprisingly amount of linkage between your addresses already, just by the way they are combined for some transactions. By reusing change addresses, you are dramatically amplifying that effect. That why this feature is hidden under the "Expert Mode."
For instance, if you always shuffle your coins between 10 addresses, then there is no question that all 10 addresses will eventually be linked, and undeniably part of the same wallet. Thus, every person you ever transact with, will have the capability to see your entire wallet balance at any time, as well as every transaction you ever execute.