Add form inputs dynamically in admin forms

[tarek.fellah]

I would like to add input forms in ajax forms, i'm searching for 2 days for a solution but i didn't find a solution.

I have an text field zip cod when the user typed 4 characters, an ajax call will be done and i would like the result (cities) to be as checkboxs and added in the form,

i tried this code 

$options = array();								
	for( $i = 1; $i <= $c  ; $i++){

	$f = "".$i."";
	 
	  $json = json_decode($htmlF)->$f->ville;
	
	 $ville = json_decode($htmlF)->$f->ville;
	  
	  $options[] = array(
			"id" => $ville,
			"name" => $ville
		);
	
	}
	//var_dump($this->fileds_temp); exit();
	//$this->fields_form =$this->fileds_temp;
	
	$this->fields_form['input'][]=	array(
										  'type'    => 'checkbox',                         // This is an <input type="checkbox"> tag.
										  'label'   => $this->l('Options'),                // The <label> for this <input> tag.
										  'desc'    => $this->l('Choose options.'),        // A help text, displayed right next to the <input> tag.
										  'name'    => 'options',                          // The content of the 'id' attribute of the <input> tag.
										  'values'  => array(
											'query' => $options,                           // $options contains the data itself.
											'id'    => 'id_option',                        // The value of the 'id' key must be the same as the key for 'value' attribute of the <option> tag in each $options sub-array.
											'name'  => 'name'                              // The value of the 'name' key must be the same as the key for the text content of the <option> tag in each $options sub-array.
										  ));
	return parent::renderForm();

I tested the $this->fields_form['input'] variable before adding it the checkbox and it retrun null, any one had an idea how to solve it or (if it's not a good solution) how to add input element in controller ajax function?

Thanks.