TYPO3 ver. 6.x - additional configuration a.k.a. `localconf_local.php`

Got it! :)

The answer is: typo3conf/AdditionalConfiguration.php if this file exists it's included automatically, to overwrite some values we need to use well-known syntax ie.:

<?php
$GLOBALS['TYPO3_CONF_VARS']['DB']['database'] = 'my_local_database';
$GLOBALS['TYPO3_CONF_VARS']['DB']['host']     = 'localhost';
$GLOBALS['TYPO3_CONF_VARS']['DB']['username'] = 'username';
$GLOBALS['TYPO3_CONF_VARS']['DB']['password'] = 'mypassword';
?>

From "What's new" SlideShare (page 33)

In addition to your own answer pay attention to a fact, that from 6.0 you shouldn't access TYPO3_CONF_VARS directly in your extension, but by getLocalConfigurationValueByPath('BE/loginSecurityLevel') and setLocalConfigurationValueByPath('BE/loginSecurityLevel', 'rsa') in \TYPO3\CMS\Core\Configuration\ConfigurationManager class.

Taken from same "What's new" SlideShare page 35.

UPDATE 15.06.15: Page 35 was included by accident to slides. Since TYPO3 6.2 ConfigurationManager is marked @internal. So, you shouldn't use ConfigurationManager in your own code. Details