How to add tab in Customer Information in Magento Admin

Step 1)
Override the file /app/code/core/Mage/Adminhtml/Block/Customer/Edit/Tabs.php,

Inside _beforeToHtml() method, add the following code:

$this->addTab('Custom',array(

    'label' =>Mage::helper('customer')->__('Custom'),

    'class' =>   'ajax',

    'url'   =>   $this->getUrl('*/*/custom',array('_current'=>true)),

));

Step 2) 

Override the file /app/code/core/Mage/Adminhtml/controllers/CustomerController.php,

Add the following code:

public function customAction()

{ $this->_initCustomer();

$this->getResponse()->setBody(

Mage::app()->getLayout()->createBlock('core/template')->setTemplate('custom/customer/tab/custom.phtml')->setCustomerId(Mage::registry('current_customer')->getId())

        ->setUseAjax(true)->toHtml()

);

}

Step 3) 

Create the file /app/code/core/Namespace/ModuleName/Block/Adminhtml/Customer/Edit/Tab/ and create Custom.php,

Add the following source code to the file:

<?php

class Custom_Custom_Block_Adminhtml_Customer_Edit_Tab_Custom extends Mage_Adminhtml_Block_Widget_Form

{

    public function __construct()

    {

        parent::__construct();

        $this->setTemplate('custom/customer/tab/custom.phtml');

    }

}

?>

Step 4)

Now, you need to create a template file.

Go to /app/design/adminhtml/default/default/template/modulename/customer/tab/ and create custom.phtml,

Write your code here.