$installer = $this;
$installer->startSetup();

$installer->getConnection()->addColumn(
    $installer->getTable('core_store_group'),
    'pp_souche',
    "VARCHAR(50) DEFAULT NULL"
);

$installer->getConnection()->addColumn(
    $installer->getTable('sales_flat_order'),
    'pp_souche',
    "VARCHAR(50) DEFAULT NULL"
);
$installer->getConnection()->addColumn(
    $installer->getTable('sales_flat_order'),
    'pp_order_id',
    "VARCHAR(20) DEFAULT NULL"
);
$installer->endSetup();