There is a way to wire both the gas and brake pots to the same input so they show up as one axis.
(got this image from a quick google search and is for a similar application, I'm assuming it is correct, but you would probably need different value resistors. This is on the list of things I never seem to get around to trying)
EDIT: I DON'T THINK THE RESISTORS ARE NECESSARY
I'd still love to know if we could solder in some extra parts and get those extra axis though.
I sold my old immersion boards. I bought two newer ones that I haven't messed with, but I don't think they have the extra axis either.