No, Microsoft Flight Simulator 2004 add-on aircraft cannot be transferred to Microsoft Flight Simulator X.

It is exactly what the name states, it is a flight simulator, it is as realistic as you can get (besides Microsoft Flight Simulator X).

Yes, you can download add-ons for Microsoft Flight Simulator 2004.

To open aircraft doors in Microsoft Flight Simulator 2004, press: Shift + E.

Yes, you can get a Dash-8 in Microsoft Flight Simulator 2004 by downloading a Dash-8 add-on. Just type "Flight Simulator 2004 Dash-8" into a search engine.