<?php class Third_Party_Model_Columncount
    {
        public function toOptionArray()
        {
            return array(
  array('value'=>'3', 'label'=>Mage::helper('ThemeConfig')->__('3 Column Grid')),
  array('value'=>'4', 'label'=>Mage::helper('ThemeConfig')->__('4 Column Grid')),
  array('value'=>'5', 'label'=>Mage::helper('ThemeConfig')->__('5 Column Grid'))
                );
    }
    ?>

<config>
  <modules>
    <My_Module>
        <active>true</active>
        <codePool>local</codePool>
        <depends><Third_Party/></depends>
    </My_Module>
  </modules>
</config>

<config>
    <modules>
      <My_Module>
       <version>0.1.0</version>
      </My_Module>
    </modules>
    <global>
        <models>
        <module>
        <class>My_Module_Model</class>
        <module>
            <party>
                <rewrite>
                    <columncount>My_Module_Model_Columncount</columncount>
                </rewrite>
            </party>
        </models>
    </global>
</config>

<?php class My_Module_Model_Columncount extends Third_Party_Model_Columncount
{
    public function toOptionArray()
    {
        return array(
        array('value'=>'3', 'label'=>Mage::helper('ThemeConfig')->__('3 Column Grid')),
        array('value'=>'4', 'label'=>Mage::helper('ThemeConfig')->__('4 Column Grid')),
        array('value'=>'5', 'label'=>Mage::helper('ThemeConfig')->__('5 Column Grid')),
        array('value'=>'6', 'label'=>Mage::helper('ThemeConfig')->__('6 Column Grid'));
    }

}?>