Seconded! I recently got the 8BitDo M30 gamepad (X-Box version) which has 6 face buttons and 4 shoulder buttons, where 2 of them are mapped to L3 / R3. But the game doesn't allow me to bind them, so I'd have to use something like Joy2Key.

I don't think it's realistic that the game will get an update after nearly 8 years on Steam. But maybe the devs will still read this, and consider implementing this for their future games, at least.