be aware that digital pads, ie old school psx pads, probably won't work with the vast majority of ps2 games that require dualshock controllers. you can try hacking 3rd party dualshocks, but you won't be able to get true analog stick functionality without using analog joysticks. also, depending on what usb adapter you get, you can have considerable lag and limited functionality.
that being said, i do currently have a working 4 player DIY playstation adapter built for the parallel port and 4 digital psx controllers. i can sell them altogether for $30ish with bare pcbs. if you want me to hack em for you, we can negotiate something according to how much effort you want put into them.