How to change a function defined inside a js module

Question

I'd like to know if it's possibile to "extend" the function named canvas_listeners that is defined inside this module and how to do it. I'm very new to javascript, I'm sorry, I'm not even sure if this is a module at all :(

plugin.js

var XYZ = (function ($, xyz) {
    'use strict';
    var xyz = {};
    xyz.canvas = {};

    $(document).ready(function () {
        canvas_start();

        function canvas_start()
        {
            //something
            canvas_listeners();
            //something
        }

        function canvas_listeners()
        {
            xyz.canvas.on('object:selected', function (options) {
                //something
            });
        }
    });

    return xyz;
}(jQuery, XYZ));

From another js file, I'd like to add to canvas_listeners something on the event object:modified. This is my attempt but obviously it doesn't fire because it's not inside canvas_listeners..

myscript.js

$(document).ready(function () {
    var XYZ = (function ($, xyz) {
        xyz.jailBox = function() {
            xyz.canvas.on('object:modified', function (options) {
                //something
            });
        }
    return xyz;
    }(jQuery, XYZ || {}));
});

I've searched but I can't find how to add my event to the original function.

Adding methods to JavaScript module pattern

http://www.adequatelygood.com/JavaScript-Module-Pattern-In-Depth.htm

Thanks for your help. Regards Ziqu

EDIT Ok, I've found a way to add a public function to the module but it still won't fire. The plugin uses fabric.js: the canvas object is created inside canvas_start() that is private as canvas_listeners() and I can't hook into both the functions. In myscript.js XYZ.canvas is: {} so I get the error XYZ.canvas.on is not a function In plugin.js xyz.canvas is: #

Still looking for a solution, any help will be very appreciated.

myscript.js

$(document).ready(function () {
    XYZ = (function ($, xyz) { //without 'var'
        xyz.jailBox = function() {
            XYZ.canvas.on('object:modified', function (options) { 
                //my function code here
            });
        }
    return xyz;
    }(jQuery, XYZ || {}));
});