This has been discussed several times on the forum, and it's already used by GSX with some internally supported airplanes (the ones with their own staircase).
Just set a door's height to 0 in the GSX airplane configuration, so no stairs will go there, and you can use the airplane staircase.
The passenger bus doesn't have any relationship with the airplane doors of which staircases you use. It will always come if the parking doesn't have a jetway, unless you explicitly disable it on that parking with the parking customization dialog.
