Impatiens need consistent 73-77F temperatures to germinate. You may be able to start them indoors until germination is complete and then transition outdoors to the greenhouse. You can grow them as long as the night temperatures in the greenhouse is atleast 55F.

Unless it is heated at night, you are probably going to wait quite a while. A greenhouse quickly looses the heat at night, and the temperatures during daytime can go very high. You should consider night heat, plus daytime venting (if necerssary).