Modifying FS2K2 Pro aircraft.

I noticed that some of the aircraft seen in FS2k2Pro are not flyable by the player.
Aircraft in question:
DeHavilland Dash-8
Boeing MD-80.
Piper.
Is there a way these aircraft can be modified so that they are "User Flyable" and if so how do I do that.
Thanks for your help.

Aircraft in question:
DeHavilland Dash-8
Boeing MD-80.
Piper.
Is there a way these aircraft can be modified so that they are "User Flyable" and if so how do I do that.
Thanks for your help.
