[Xeno86k]

The original Motif, and ES Motif had expansion slots that Yamaha still sells cards for. One being a FM Synthesis six operator based 16-polyphony card labeled the "PLG150-DX". A screen shot of the expansion card looks like it has a YMP705-F on it, but the fine print from them about using patches from their FM keyboards "Some voices may sound slightly different when played back on the PLG150-DX" makes me a little suspicious.

However with the launch of the Motif XS line in 2007 these slots where removed from the keyboard, and i don't know if the XS Rack version has the expansion slots back in. So you would have to find a used original or ES model Motif keyboard or rack if you want to go this route.