Do you mean 3rd party or default aircraft? Most 3rd party FS98, CFS1 & FS2000 aircraft will work fine in CFS2 without any conversion. They might need a simple tweak to display on the menu. Please read this. http://www.simviation.com/lair/aliasing1.htm

For the CFS1 defaults try C.A Simpson's ready-converted versions. CFS2 Axis & Allied Expansion Packs here. http://www.simviation.com/cfs2aircraft1.htm

PS. For external weapons to display & function on 3rd party aircraft patch the MDL file with the MDL Converter Tool.
http://www.microsoft.com/games/combatfs2/articles_SDK_mdlconverter.asp
Don't use the Optimize Graphics option.