adding tax_query to $query_string

I need to adjust the query in the loop.php by adding tax_queries based on $REQUEST params. i tried something like this

$argsExtended = array(
	'tax_query' => array(
		array(
			'taxonomy' => 'my_custom_tag',
			'slug' => 'slug',
			'terms' => array( 'my_tag_slug' ),
			'operator' => 'in',
		),
	)
);
$args = array_merge( $wp_query->query_vars, $argsExtended );
query_posts( $args );

but this doesn’t seem to work. Any ideas?