The usable keys without extending the Input script are:
Arrow keys, plus the additional keys you see when you hit F1 in game.
You could map W,A,S,D to UP,LEFT,DOWN,RIGHT in a script, but not in the keyboard control menu (F1)
However, W is currently used in the default scripts for "Page Down" in several of the menu scenes.
So, you'd have to map W to something else first. (Z?)