There is no conversion but the model should work in FSX. The problem with putting FS9 files into FSX is normally the cockpit gauges. The aircraft that I've given you doesn't come with a cockpit panel but aliases(connects to) the default B747 panel. I've never tried it in FSX but it should work. There isn't that much out for FSX, that's all that I could find.
PS You have really gotten me into the Convair 880. I've researched the aircraft, downloaded it with two paint schemes(FS9). I have found a panel, engine smoke effect and sound files for the Convair 880 but it's all FS9.