Sorry I thought you were saying the mouse wheel appeared as a regular analog input.
If you set a key or button to shift between control modes you can use any button currently assigned to something else for map zoom. Copy mode 1 to mode 3 and set the button to shift to mode 3 when you're in mode 1 and in mode 3 set it to shift back to mode 1. This lets you shift any controller or button to a different function and back again with a button press.