Thing is that EZ2ON only works with direct input from an "real" keyboard. No Joysticks/Gamepads.
So this way the Game can protect himself from cheating.
Cheating? Well you could write for example with GlovePIE an small script which detects the notes on the chart and press/activate the button in the right moment.
Anyway, that are just my thoughts.
If you really want to use your Beatmania Controller, then you could buy an
Keyboard Encoder and solder the Pushbuttons to it.