http://stackoverflow.com/questions/5961290/adding-attributes-to-customer-entity/5962237#5962237
$installer = $this;
$installer->startSetup();
$setup = new Mage_Eav_Model_Entity_Setup('core_setup');
$entityTypeId = $setup->getEntityTypeId('customer');
$attributeSetId = $setup->getDefaultAttributeSetId($entityTypeId);
$attributeGroupId = $setup->getDefaultAttributeGroupId($entityTypeId, $attributeSetId);
$setup->addAttribute('customer', 'your_attribute_code_here', array(
'input' => 'text',
// 'type' => 'int',
'type' => 'varchar',
'label' => 'Some textual description',
'visible' => 1,
'required' => 0,
'user_defined' => 1,
));
$setup->addAttributeToGroup(
$entityTypeId,
$attributeSetId,
$attributeGroupId,
'your_attribute_code_here',
'999' //sort_order
);
$oAttribute = Mage::getSingleton('eav/config')->getAttribute('customer', 'your_attribute_code_here');
$oAttribute->setData('used_in_forms', array('adminhtml_customer'));
$oAttribute->save();
$installer->endSetup();