I use one of these pedals with my Kurzweil PC3K8
http://www.amazon.com/M-Audio-SP-Triple-Electronic-Sostenuto-Functions/dp/B01877I1YY
It works splendidly with my Kurzweil, but my keyboard has three independent 1/4" switch pedal jacks.
It may be possible however to kludge a cable or adapter to use this with your Kurzweil Mark 5.
It shouldn't take much more than basic electronics skills to wire up an adapter cable, but if you don't personally feel up to the task you could always contract a service tech to do the job for you.
It's simply a matter of tracing back the open and closed resistance in Ohms of each cable of the ribbon for when each switch is depressed and then translating that electrically to a short adapter cable that mates up with the pedals as well as the keyboard rribbon connector.
One caveat, however, the pedal I listed does not do half damping.
If that is a feature of the Mark 5 pedal you may have to learn to live without it.
Gary