Currently, there's no way to specify a custom session store. You don't have to impelment a new one; is already exist, see CallContextSessionStore.cs.
You must not use an other ISessionStore than WebSessionStore as you may suffer lost of session. -- Cheers, Gilles <a href="http://www.amazon.com/gp/registry/6JCP7AORB0LE";>Wish List</a>
