How can I add a button to quicktags-toolbar?

Try adding the following code in your function.php

add_action( 'admin_footer-post-new.php', 'wpse_64665_add_quick_tag' );
add_action( 'admin_footer-post.php',     'wpse_64665_add_quick_tag' );
function wpse_64665_add_quick_tag() { ?>
    <script type="text/javascript">
        edButtons[edButtons.length] = new edButton( '„“', '„“', '„', '“', '' );
    </script><?php
}