There isn't anyway to change the keybinding - it is sealed to the scroll wheel. You can change the zoom though with a macro. It depends on what you really want to with the zoom.
Useful addon,
AutoZoomOut.
I can't seem to quickly find the command for making your own macros, but it is something like
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Source code
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1
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/run setcameradistance(x)
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so you could have a few different macros for different distances. One thing to watch out for though is that this is the suicide macro - zoom out too far and you will die, though without debt or damage.