This sort of thing COULD be done, but really it is more trouble than it is worth. It gains no real functionality over just running Mame, and you can easily fry your Tetris board, computer, and/or power supply if you mess up your wiring.
Also, please don't mess up your original control panel by adding buttons. Make a new panel if you decide to Mame, and just leave your old one unharmed. Or even better, just stick with the controls that are on the panel, and simply run games that work with those controls. (No matter what controls you have there are a ton of games that can use them).