Adding spirit form to demons in DF.

I was wondering how you would add spirit form to the demon monsters in the DF books. The way I imagine demons is that they are evil spiritual beings that can materialize to physically attack the PCs. I imagine they would likely spend most of their time as spirits wandering dungeons and the lands outside them at night in their spiritual form. Then when they decide to attack they quickly materialize and then they are exactly the same as the stats in the DF books. I don't want the demons to be able to switch back and forth from material to spirit all of the time however. They can materialize and then they attack until they are almost dead and then the turn back to spirit at the last minute leaving behind a sulfurous cloud of foul smoke. In the spirit is then destroyed they are then sent back to Hell.

Also demons can possess a person by placing their spirit inside the body of a living PC. So how is the best way to handle this addition to the demons in the DF books?

Adding spirit form to demons in DF.