Most likely it already IS PS/2. The trick then is figuring out what the pinout on it is, and make a connector.
You should first see if you can find the info on the web. If you can't you might be able to figure out some pins, like power and ground, just by looking at the PCB, and simply try the data and clock in different combinations. That's what I did for a touchpad for which I couldn't find the pinout.