You'll probably need a micro controller to build something like that. I don't know off the top of my head how the PS2 controller ports worked (wiring/protocol), but the Wii stuff that you plug into the Wiimote uses the I2C bus, I think with a custom connector. So a simple electric adapter will not work, you'd need something like a SNES to USB adapter box, but more complicated. You would not only have to convert the plug (that's the easy part) but also have to implement the handling the Wiimote does and probably some protocol translation. It should be possible with a FPGA and some soldering/custom board with connectors, but I have never seen something like this sold commercially. Could be because of the missing L3 and R3 "buttons".