Create a Magento customer attribute and add it to admin forms

customer_attribute.php

<?php

/* @var $installer Mage_Model_Customer_Entity_Setup */
$installer = $this;

/* @var $eavConfig Mage_Eav_Model_Config */
$eavConfig = Mage::getSingleton('eav/config');

$installer->startSetup();

$installer->addAttribute('customer', 'an_attr', array(
    'type'     => 'text',
    'label'    => 'An Attribute',
    'visible'  => false,
    'required' => false
));

// look for form codes scattered around the mage codebase set using $model->setFormCode()
// here are some customer examples
$forms = array(
  'adminhtml_customer',
  'adminhtml_customer_address',
  'customer_address_edit',
  'customer_register_address',
  'customer_account_create',
  'customer_account_edit',
);

$eavConfig->getAttribute('customer', 'an_attr')
    ->setData('sort_order', 126)
    ->setData('used_in_forms', $forms)
    ->save();

$installer->endSetup();