So I get pretty ambitious with the things I do with my M. It's a 2007 M35 base. I love the car, but I want to get more out of it and the 5-speed is holding me back. I'm not really well-versed in the transmission side of things, but would it be possible to swap out the 5-speed for a 6-speed manual from a G? And how could I go about it? Thank you!

While I have no idea how to go about it, it is certainly possible given enough skill and/or money. One of the famous drivers from Formula Drift used to use an M45 and he did a manual swap on it.