var anOtherOnZoomFunction = function(info) {
    console.log(info);
};

$.axZm.userFunc = $.fn.axZm.mergeCallBackObj($.axZm.userFunc, {
    onZoom: anOtherOnZoomFunction
});

// or like this when AJAX-ZOOM is already initialized:
$.fn.axZm.addCallback('onZoom', anOtherOnZoomFunction);