Everything I can see for it on the Arch wiki indicates the filesystem would need to be unlocked and after an update your changes would be wiped out. Alternatively brew.sh would potentially let you install xmodmap to do it although, without testing either method, I can't say if the changes would even follow through from Desktop to Game mode.
However, in Game mode you can open these menus with Ctrl+1 for the Steam button and Ctrl+2 for ellipsis button.