Hello Howdoin, and welcome to the forums.
To answer your question, NEC unfortunately never produced a controller extension cable for the PC-FX. If you feel adventurous, I would think that you could just cut the controller cable in half, splice in 3 feet of wire in the middle and that should be it - however I have never done it, so do at your own risk (would be a heck of a lot easier than trying to make your own extension, that's for sure.)