Extending an existing jQuery function

Sure… Just save a reference to the existing function, and call it:

(function($)
{
    // maintain a reference to the existing function
    var oldcss = $.fn.css;
    // ...before overwriting the jQuery extension point
    $.fn.css = function()
    {
        // original behavior - use function.apply to preserve context
        var ret = oldcss.apply(this, arguments);

        // stuff I will be extending
        // that doesn't affect/change
        // the way .css() works

        // preserve return value (probably the jQuery object...)
        return ret;
    };
})(jQuery);