Last I checked, you can replace any of the icon files with your own and it will use those where-ever the old one was used. To my knowledge that is the only way it can be done -- either replace every instance of an icon or not at all; it is not currently possible to just 'add' more icons to the selector in the macro window, although you can use any in-game icon that you have so long as you have a source to 'drag' the icon from (a spell, item, etc.)
They (SOE) even recently added some more general icons to the selector there.
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