You may connect an analog joystick to your SY99. It works with a little rewiring, so maybe it won't work with your computer anymore. Both the SY pedals (not switch!) and the breath controller jacks work with an external poti as a voltage divider (is this the correct technical term?). 

In a similar vane.... can a breath controller be used with the foot controller or foot volume inputs on an SY85? 

Just in case someone wants to modify a joystick for the pedal inputs: to play it safe, try to get a joystick that allows trimming the zero-position. Otherwise you may have problems to get the full controller range (0-127). The trimming control on my joystick is via two small sliders besides the stick area. Thanks to Rheinhold reminding me of the problem.