There's no real way to customize it, but you can set up an event that captures a button and does an event.. Such as the number 0 or the Period key.

The number pad is used for directional movement by default anyway, turn off Num Lock to use it, otherwise it considers it a "4" instead of Left, for example. The 0/Insert key is the Cancel key, and the Enter is for.. Enter. Not too difficult, just remember to turn of the Num Lock..

It's JOYSTICK compatible, I know that much. My best educated guess is that "PS2-to-PC" adapter sets it up as a standard joystick, so you can set up the buttons themselves and calibrate it through Windows. If that's true, RPG Maker shouldn't have any trouble with it.