Unless somebody else knows a way around it, you cannot register this key from Impulse on Steam.
Just thought I'd let anybody know. The Impulse client, whatever its called, did update the game to the latest patch when I installed it.
I've had the impulse/stardock client for a while, have 8 or 9 games on it, not as proficient with it as Steam yet, but you can have it search for updates, etc, so its not totally useless.
I played Warband for a little bit, but it ended just like most of my attempts to play the first Mount and Blade, a Crash to Desktop after about a half hour or so. It does have its quirky charm, like the first one.