Thank you, **@yvan**, for answer. Links you added are good.  
But there are better documentation in source code comments. If you want to understand how it works you should look through files `includes/ajax.inc` and `misc/ajax.js`.  
My problem was solved like this:  
<!-- language: lang-js -->

    (function($) {
	Drupal.behaviors.MyModule = {
		attach: function (context, settings) {
			
			// Overwrite beforeSubmit
			Drupal.ajax['some_element'].options.beforeSubmit = function (form_values, element, options) {
				// ... Some staff added to form_values
			}
			//Or you can overwrite beforeSubmit
 			Drupal.ajax['some_element'].options.beforeSerialize = function (element, options) {
				
				// ... Some staff added to options.data
				// Also call parent function
				Drupal.ajax.prototype.beforeSerialize(element, options);
			}

                        //...

These functions are calling in [jQuery Form Plugin][1].


  [1]: http://jquery.malsup.com/form/