I'm trying to have a situation in which i'm trying to get past a certain barricade by completing a certain task, but I can't figure out how to either insert a new walkable area after completing a task, or make an object non-walkable then move or remove it.
You are probably looking for the RemoveWalkableArea and RestoreWalkableArea functions - you can't add a new walkable area, but you can create 2 from the start and turn one off until such a time as you want it turned back on :).
See here (http://www.americangirlscouts.org/agswiki/Room_functions#RestoreWalkableArea) and here (http://www.americangirlscouts.org/agswiki/Room_functions#RemoveWalkableArea) in the manual. Keep in mind that you have to keep removing walkable areas each time you load a room, as their state is reset on leaving the room.