The only way to do it which I know is to use a computer.

I used to play my PSR-3000 with an external keyboard and I must say that it wasn't very comfortable. All the controls get farther, and so does the speakers, as a result your ears kinda get out of a proper sound image.

A few years ago someone made a remark on this forum that the PSR-9000Pro didn't sell well, so Yamaha has been trying not to make the same mistake since then.