It's not just a button, but you have to link the search results to the result page of zoomsearch. That requires an additional SLP (layout page) that zoomsearch can use. And the search boxes have to be linked to this result page. But that means that the logic of the layout has to be updated. When you start a search, items are shown and hidden based on settings. That will have to be updated as well.
The best way to start is to look at the full JavaScript as provided by Adobe: Learn how to download and use new screen profiles and layouts in RoboHelp You can then write an extension to hook in zoomsearch.