If you really want to do this(not sure I would!),
Rightclick the icon
Then Righclick again
Another option might be to add your new button to the toolbar and copy the button image of the exixting one (and perhaps edit it slightly to make it obvious it is different?)
First put YOUR button on the toolbar with whatever the button image is at the moment
"Drag the Yellow Smiley to the toolbar"
"Right Click the Yellow Smiley and Assign your Macro
Don't press "Close" yet
Right click the "new workbook" icon and "Copy Button Iimage"
Right click your yellow smiley and "Paste Button Image"
Right click your now not yellow smiley and "Edit Button Image"
Of course, this may not be what you want at all!