Change scope of validation rules of customer attribute property from global to website

Question

How can I change the scope of the customer attribute property "validation_rules". This attribute exists in customer_eav_attribute (containing min_text_length and max_text_length).

Adding it to customer_eav_attribute_website does not work. Scope is still "global". Where do I have to customize?

$installer->getConnection()->addColumn($installer->getTable('customer/eav_attribute_website'), 'validate_rules',
    'text DEFAULT NULL');

Any ideas?