First let me clarify that you dont need to zoom in order to make a character "active", just simply mouse over.
If you do want to use a key to zoom a window, do something like this:

<<Barelan>>
Max
This will zoom my hunter
The command to un-zoom:
Std

You could then assign whichever key you would like to these commands.

You could even just use one key with a "Get" command:

Get, ZoomWho, who would you like to zoom 1-5
Chan %ZoomWho%
Max

This would ask you which window to zoom.

A Slightly more sophisticated version of this would be:

Get, ZoomWho, who would you like to zoom 1-5, 6 to un-zoom
if %ZoomWho% = 6
Std
else
Chan %ZoomWho%
Max
endif

This would allow you to have the un-zoom on the same key.