Is it possible to add extra walking animations? Or is that hard coded?

You can create your own animation file that spans all animations possible, or you could edit the walking portion of base_anim.nif and the associated files, which will give all NPCs your new walking animation. If you just want the walking animation for the player, copy base_anim.nif and it's associated files, edit the walking portion, and resave it as something unique. Then specify your animation in the properties window for player (that'd be you) in the CS. I'm not a modeler, and I've only dabbled a little bit in animations (using them, not making), so I'm not the best one to answer your question.