Strictly speaking, we can't guarantee that you'll be able to register the expansion key in steam, but as Draco18s said the expansion already works with the steam game.
That said, if you don't have AS through steam (or registered through steam, if that becomes possible) and need to do a full reinstall of AIW you might need to grab the AS expansion installer separately (from our website, for instance). And steam would not remember the key for you, but you could use it just fine.
Anyway, after how long it took for AVWW keys to be registerable in steam I'm trying to emphasize that we're offering no
guarantees or promises on being able to register a non-steam key in steam