It is not recommended to add/modify or do changes to any core files . Better you make a new module to add an extra column .
You have to mention correct version for module upgrade in app/code/local/your/module/sql/your_module_setup/upgrade-0.1.2-0.1.3.php
file. (This means your upgrade the module version from 0.1.2 to 0.1.3). If your are not using upgrade script, remember to define <resources>
in module config.xml
and the setup script name is mysql4-install-0.1.0.php
Below is Mysql setup script file - upgrade-0.1.2-0.1.3.php
<?php
ini_set('display_errors', '1');
$installer = $this;
$installer->startSetup();
$installer->getConnection()
->addColumn(
$installer->getTable('newsletter/subscriber'), //Get the newsletter Table
'your_field_name', //New Field Name
array(
'type' => Varien_Db_Ddl_Table::TYPE_TEXT, //Field Type like TYPE_INTEGER ...
'nullable' => true,
'length' => 255,
'default' => 'Some thing default value',
'comment' => 'Your field comment'
)
);
$installer->endSetup();
?>
and after that change app/code/local/your/module/etc/config.xml version for example
<config>
<modules>
<NameSpace_ModuleName>
<version>0.1.3</version> <!-- if upgrade script version is 0.1.3 -->
</NameSpace_ModuleName>
</modules>
<global>
<resources>
<NameSpace_ModuleName_setup>
<setup>
<module>NameSpace_ModuleName</module>
<class>Mage_Catalog_Model_Resource_Setup</class>
</setup>
<connection>
<use>core_setup</use>
</connection>
</NameSpace_ModuleName_setup>
</resources>
</global>
</config>