is there a way to make the dust sprial down to the blackhole
Good question... Someone please correct me if I'm wrong, but bodies don't typically spiral down because of gravity.
Open up 'Saturn & its Moons' then increase the mass of Saturn to around 1 Jupiter. The rings will appear to 'spiral' into the planet but what doesn't crash will assume stable elliptical (oval) orbits.
You could get a spiral if the rings or moons experienced some sort of drag where they continually lost a very small fraction of their velocity as they orbited the planet, star, or black hole.
I may add this feature (ability to add some kind of drag to a system) in a future release.
Does that answer your question? I'm sorry I don't have a more immediately satisfying answer.