Reassigning a modifier key in a single app

Hi there,

I'm used to working in Logic Pro, Adobe Suite and various other pieces of software, and generally, it seems that I can zoom laterally or vertically by using gestures on the mouse while depressing the Option/Alt modifier key.

However, I've recently been working a lot in Reason Studio's Reason and it uses Command as the zoom modifier. It's driving me a little crazy. I've looked at System Preferences > Keyboard > Shortcuts, but this only seems to work with dropdown menus. I wonder if anyone could suggest a way to reassign this modifier key in Reason.