I do not think that FSX has any sort of portable Airstair feature. The only thing I can think of is the one on the POSKY ERJs or a static staricase as part of a scenery pack.
The only ones that pull up to the plane that I know are in FS9, there are a few packs that have the Air Stair feature set as like, tailhook down or something, and it rolls up to the jet.