You can either:
1) make a copy of movement.wdl and paste it into your game folder, and then alter the player_intentions() section in that (I say make a copy so that you don't mess with the template)
2) check out Keebo's great key definition script on his site (sorry, can't remember the URL).
3) or 3, even better yet, wait for the next update of A5 (hopefully out later this month) - the next update will include new commands that make user definition of keys a cinch (check out "known bugs and upcoming features" in the links section at the bottom of the forum.
Hope that helps!
Keith