Add functionality to existing wp plugin

I have an existing plugin which I purchased. I would like to add some functionality to it which I need for my site. The plugin is used to create pop-ups on my site. There is no functionality to have pop-ups triggered when users exit my site (clicked external link, closed window, closed browser). I would like this to be added to the plugin. To see pop-up in action go to: <[login to view URL]> When you access the page above, my pop-ups are triggered just fine. Need pop-ups to be also triggered during exit. I want the ability to choose which pop-up(s) get displayed at exit. ## Deliverables The editor/creator for new modal windows needs a new section added to it where user can set option for modal window to appear during site exit. It would be nice if user can select what exit event(s) (selection box for click on external link, closed browser window, closed browser) triggers the modal window to appear. Once user saves newly created modal window or edited modal window, then exit pop-up should be triggered during exit on pages/posts where the shortcode or html code for the modal window (with exit trigger selected) has been placed.