On a FSX plane, you will need to save it into the Gmax 'FS2004' version. (There are two, FS2004 SDK and FSX SDK). Next, you would delete various animations for things like props, tires, etc, if their animation name tags do not have 'key' in them (animated). Some animations in FS9 are auto-generated, like tire rotation, so deleting the tire animation Key Frame points and naming them properly for their animation tags will make them work in FS9.
It may be faster to bring a FSX model into FS9 format then to convert a FS9 plane to FSX. Simply create the new Materials, change the animations, rename parts with proper animation name tags, and export.