Possible bridge you can walk under? (Probably model editing question)

There's a doodad I an thinking of, one that many people use in maps for cave entrances and so forth. It has a walkable top, and units can enter it from the side slightly but not all the way. If the model itself were edited could it be made so a unit could simply pass through it completely, like by stretching the hole to the other side?

nope. the way WC3 works, you cannot go under the bridge, just over it. The pathing map is not on 3 dimensions, just 2. Because of that all areas in the 3rd dimension are limited to ontop of that pathing.