You can't set the exact path because, of course, the path can only be calculated dynamically, since it cannot be always the same, because it changes depending on the airplane type and how it's parked.
Usually, what can be done to improve this, is setting the carts starting position, but that's only valid for vehicle used on Deboarding. For Boarding, with vehicles coming from far away, what affects the path, is how the airport is designed and how well (or badly) the vehicle paths are made (if they are even there).
