Yes and no, there are aircraft carriers in Microsoft Flight Simulator X: Acceleration (add-on) and Microsoft Flight Simulator X: Gold Edition.
No, Microsoft Flight Simulator 2004 add-on aircraft cannot be transferred to Microsoft Flight Simulator X.
No, you cannot add aircraft to the demo of Microsoft Flight Simulator X.
Yes, you can get a Concord for Microsoft Flight Simulator X from websites that make add-on aircraft for Microsoft Flight Simulator X.
To open aircraft doors in Microsoft Flight Simulator 2004, press: Shift + E.
Microsoft Flight Simulator X: 18 Microsoft Flight Simulator X: Deluxe Edition: 22 Microsoft Flight Simulator X: Acceleration (add-on): 25 Microsoft Flight Simulator X: Gold Edition: 25
You can get more aircraft in Microsoft Flight Simulator X by buying/downloading expansion packs or by buying/downloading specific aircraft.
You can't, the coding on the planes is different. Most of the planes you find in Microsoft Flight Simulator 2004 are available in Microsoft Flight Simulator X.
There is no need to buy Microsoft Flight Simulator X: Deluxe Edition as Microsoft Flight Simulator X: Acceleration contains everything from Microsoft Flight Simulator X: Deluxe Edition and more.
there are many sites that have free downloads for many of the aircraft types
No, you cannot use the add-ons for Microsoft Flight Simulator 98 in Microsoft Flight Simulator X.
Flight Simulator X is by Microsoft, the full name is Microsoft Flight Simulator X.
Microsoft Flight Simulator was created in 1982.