Magento – Dynamically add “Add More” button to magento configuration

I have created a module which allows to add menu items dynamically from admin panel configuration. For this, I have added a dynamic field with "Add more" button.
It works perfectly and the menu I have created is being displayed on top menu. Now I want to add submenu to that menu item.

You can see it in the image.
I have this code in my form field block

public function __construct()
    {
        $this->addColumn('menutitle', array(
            'label' => Mage::helper('adminhtml')->__('Menu Title'),
            'style' => 'width:120px',
            'class' => 'input-text required-entry'
        ));
        $this->addColumn('link', array(
            'label' => Mage::helper('adminhtml')->__('Link'),
            'style' => 'width:220px',
            'class' => 'input-text required-entry'
        ));

        $this->addColumn('order', array(
            'label' => Mage::helper('adminhtml')->__('Order'),
            'style' => 'width:40px',
            'class' => 'required-entry input-text validate-number'
        ));

        $this->_addAfter = false;
        $this->_addButtonLabel = Mage::helper('adminhtml')->__('Add Menu Item');
        Mage_Adminhtml_Block_System_Config_Form_Field_Array_Abstract::__construct();
    }
    protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
    {
        $this->setElement($element);
        $html = $this->_toHtml();
        $this->_arrayRowsCache = null; // doh, the object is used as singleton!
        $html ='<div id="myeditableitem">'.$html.'</div>';
    return $html;
    }

I have tried adding a button in form but without success. I dont have any idea where or how to start. Please help.