As you are trying to create attribute for two different entities use following code in config.xml
<config>
<modules>
<Namespace_Module>
<version>0.1.1</version>
</Namespace_Module>
</modules>
---
---
<resources>
<namespace_module_setup>
<setup>
<module>Namespace_Module</module>
<class>Namespace_Module_Model_Resource_Setup</class>
</setup>
</namespace_module_setup>
</resources>
In Setup.php file write following code.
class Namespace_Module_Model_Resource_Setup extends Mage_Customer_Model_Resource_Setup
{
}
Then after create two separate installer & upgrade files
install-0.1.0.php
$installer = $this;
$installer->startSetup();
$installer->addAttribute('customer', 'organisation_id', array(
'input' => 'select', //or select or whatever you like
'type' => 'int', //or varchar or anything you want it
'label' => 'Organisation ID',
'visible' => 1,
'required' => 0, //mandatory? then 1
));
$installer->endSetup();
upgrade-0.1.0-0.1.1.php
$installer = $installer = new Mage_Sales_Model_Resource_Setup('core_setup');;
$installer->startSetup();
// now here write your code to create attribute
$installer->endSetup();