Not sure why that wouldn't work. I just tried it myself (did exactly what you said) and it worked fine. You said "it's not autoloaded" but you didn't say exactly what happened. Did you get an error? What code did you use to call the class?
As a side note, if you're writing your own customizations you shouldn't add your own classes to the core Q2A namespace. If you're writing a plugin your should keep your code within the plugin folder.
Having said that, currently there isn't a standard way to autoload custom classes yet. You could write your own autoloader similar to the Q2A core one (at the top of qa-base.php).