I don't think it can be easily done - the engine works rather differently and you can't just take the texturing component from it and use it in FSX. The only way would be to use it complete for all scenery rendering, leaving the system's simulation on FSX. I don't know if FSX allows such a mode, though.
Anyway, the effort needed would be still very high, probably comparable to making an entirely new simulator. And the result would be worse. And I doubt there will be anybody willing to invest in such a project for other reasons as well.