can do what you desire I'm pretty sure.
Here's an article in Ubuntu on using xinput and imwheel
. That should be a good point in the right direction.
I've never done what you're attempting to do but it sounds interesting. Please report back to us with your results whether successful or unsuccessful so we all benefit.
Before You Proceed
Before any further editing to xorg.conf or .imwheelrc, it might be helpful to check your mouse button mappings.
Run imwheel -c from a terminal window for imwheel's graphical UI. Pressing the GrabWheelAction button and then one of your mouse's buttons should cause imwheel to show the button's default mapping. In practice, this seemed somewhat inconsistent, but results may differ depending on your particular mouse.
I google searched "xorg modify mouse buttons".