Add a Custom Field in Comment Box AFTER text area BUT BEFORE Send button

Filter comment_form_defaults and add your code to the textarea.

Sample code, not tested:

add_filter( 'comment_form_defaults', 'wpse_120049_extend_textarea' );

function wpse_120049_extend_textarea( $args )
{
    $args['comment_field'] .= '<p>Extra text.</p>';
    return $args;
}