What I have done is: when you download a plane, b4 you put it in fs2k4, go into aircraft.cfg file in the main folder of the plane, under flightsim.# (0,1,2,3), go down to the line "ui manufacturer=Boeing" and change Boeing to the airline of your choice and the airline will show up as a manufacturer in the select aircraft menu. Also change the "title=boeing 737" to something different for each version that you have.
Here is an example:
description=One should hardly be surprised that the world's most
hope this helps...