I want to use an expression pedal, a sustain and one or two other footswitches with my s950. Can I use a multi-pedal or a/b switch.

Cass, you can assign the two pedal inputs to do anything you wish, and to perform multiple functions, just assign the pedal functions to registrations. Lots of options using this technique.

Gary

You can add a footswitch with a MIDI Solutions Footswitch Controller F1 or 8 footswitches with the F8. It's pricey though, and the keyboard doesn't provide enough power for it if your equipment has come out of the cold.

As much as I like my S910, the limitation of multiple assignments available at any one time, puts it behind my other keyboards in that respect.

That's one of the reasons I moved away from S950. I use a Peak midi controller with PA900. It is now available because the SD7 will accept the Ketron pedals.
Don't know if it will work with Yamaha because their midi implementation leaves a lot to be desired.