The official/technically correct answer is to flip the mesh (not the model, the mesh, just that particular NiTriShape or maybe node) relative to the model's origin. Or the origin relative to the world, one of the two. Maybe both combined.
There's a way to do it, because doors always swing on a particular local axis, so if you flip the local axis, the door will swing the other way. I don't remember the details, alas.