Add index in existing index in magento2

I want to add an index to an existing index using Setup/UpgradeSchema:

 $connection = $setup->getConnection();
        $tableName = $setup->getTable('sales_order_grid');
        $indexName = 'sales_ordergrid_netsuite_id';
        $columnName = 'netsuite_id';

        $connection->addIndex(
            $tableName,
            $indexName,
            $columnName,
            MagentoFrameworkDBAdapterAdapterInterface::INDEX_TYPE_FULLTEXT
        );

It’s creating new insted of adding to existing index.
How can I add an index in existing(“FTI_65B9E9925EC58F0C7C2E2F6379C233E7”) index?

Thank.