How to Extend Plugin

[Alex]

I would like to add some functionality to the Bootstrap plugins
without actually modifying the plugins.  I am new to working with
JQuery plugins and quite get it to work.  My code looks like this:

  $.extend($.fn.modal, {
        showFooterMessage: function (message) {
            alert("Hey");
        }
    });

  $(this).closest(".modal").modal("showFooterMessage");

But the error is that the object has no showFooterMessage so obviously
I am extending it incorrectly.  Can anyone suggest the appropriate
approach?

Thanks,
Alex