It tricky. There are several guides on the Internet which shows you how to convert a PS2 controller to parallel, like this guide: http://www.emulatronia.com/reportaje...xeng/index.htm
, however, it requires a number of components, and your time. And to add, this mod requires several programs in order for the controller to be functional, it isn't USB (unless you invested in a parallel DB-25 to USB adaptor, which wouldn't be money wise) and chances are this type of mod will have compatibility issues.
If you were converting an XBOX controller to USB, it would be so much more easier.
My advice would be to invest in a PS2 controller to USB adaptor.