Add new tab page in admin category - Magento - magento

Question

I'm looking for to add a new tabpage under category in magento admin, which loads the category grid. Idea is to associate multiple categories to one category and on the front end, when you click on that particular category, it should show products from all the associated categories.
Is there any better solution then create a new tabpage with categories grid?
EDITTED
Right... I have realised I could add new fields in category with install script. Now in my setup script I am doing something like this:
public function getDefaultEntities()
{
return array(
'catalog_category' => array(
'entity_model' => 'catalog/category',
'attribute_model' => 'catalog/resource_eav_attribute',
'table' => 'catalog/category',
'additional_attribute_table' => 'catalog/eav_attribute',
'entity_attribute_collection' => 'catalog/category_attribute_collection',
'attributes' => array(
'men_categories' => array(
'type' => 'int',
'backend' => '',
'frontend' => '',
'label' => 'Men Categories',
'input' => 'select',
'class' => '',
'source' => 'pushon_config/config_source',
'global' => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
'visible' => true,
'required' => false,
'user_defined' => true,
'default' => 0,
'searchable' => false,
'filterable' => false,
'comparable' => false,
'visible_on_front' => false,
'unique' => false,
'group' => 'Associated Categories',
),
),
),
);
}
In my source.php in doing this:
class PushOn_Config_Model_Config_Source extends Mage_Eav_Model_Entity_Attribute_Source_Abstract
{
protected $_options = array();
public function getAllOptions($withEmpty = true, $defaultValues = false)
{
$option = Mage::getResourceModel('catalog/category_collection');
foreach($option as $id){
$this->_options[] = array('value'=> $id->getData('entity_id'), 'label' => $id->getData('name'));
}
return $this->_options;
}
}
Now when I run this it shows me error message:
a:5:{i:0;s:163:"Error in file: "C:\wamp\www\vhosts\staging.domainname.com\httpdocs\app\code\local\PushOn\Config\sql\pushon_config_setup\mysql4-install-0.1.0.php" - Wrong entity ID.";i:1;s:1148:"#0 C:\wamp\www\vhosts\staging.domainname.com\httpdocs\app\code\core\Mage\Core\Model\Resource\Setup.php(390): Mage::exception('Mage_Core', 'Error in file: ...')
I am just trying to load all the categories. Any idea why this is happening?