As far as I know, native FSX aircraft cannot be converted to work in FS9, at least not without a complete rework of the source files for the model in the software that it was designed in. You'd have to get the original modeler to do it, or get the source files from them.
It's just like you can't get an FS9 plane to work in FS98, and so on. FSX seems to have even less backward compatibility than earlier versions of FS.