require_once MAGENTO . '/../app/Mage.php';
Mage::app();

$newFields = array(
    'custom_attribute' => array(
        'type'              => 'text',
        'label'                => 'Customer Custom Attribute'
    )
);

$entities = array('customer');

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');
foreach($newFields as $attributeName => $attributeDefs) {
    foreach ($entities as $entity) {
        $setup->addAttribute($entity, $attributeName, array(
            'position'          => 1,
            'type'              => $attributeDefs['type'],
            'label'             => $attributeDefs['label'],
            'global'            => 1,
            'visible'           => 1,
            'required'          => 0,
            'user_defined'      => 1,
            'searchable'        => 0,
            'filterable'        => 0,
            'comparable'        => 0,
            'visible_on_front'  => 1,
            'visible_in_advanced_search' => 0,
            'unique'            => 0,
            'is_configurable'   => 0,
            'position'          => 1,
        ));                
    }
}

OR

<?php

$installer = $this;

$installer->startSetup();
    
$installer->addAttribute('customer', 'interested',array(
	'type'      => 'varchar',
	'input'		=> 'text',
	'visible'   => true,
	'label'     => 'Interested sales',
 	'global'    => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
	'required'	=> false
)); 

$installer->endSetup();

http://stackoverflow.com/questions/5185727/magento-add-custom-input-field-to-customer-account-form-in-admin

The fastest way is to create a php file and access it through browser add the following content to file.

Magento add custom input field to customer account form in admin

customer, magento

PHP