tsc_a

@mattifesto

Caveats: This will exclude posts that don’t have a popularity custom field, but for people that know every post will have some value for a custom field, it’s a much easier and less fragile method of sorting by a custom field.

I definitely can’t live with that, so I looked for another solution.
Finally I asked a more “pro”-person, who came up with the following:

add_filter('posts_join', 'new_join' );
function new_join($pjoin){
	if(is_category()){
		global $wpdb;
		$pjoin .= "LEFT JOIN (
SELECT *
FROM $wpdb->postmeta
WHERE meta_key =  'add_year_value' ) AS metasort
ON $wpdb->posts.ID = metasort.post_id";
	}
	return ($pjoin);
}

 add_filter('posts_orderby', 'new_order' );
function new_order( $orderby ){
	global $wpdb;
	if(is_category()){
		$orderby = "metasort.meta_value ASC";
	}

 	return $orderby;
}

Articles without the “add_year_value” -custom field also show up now.
Hope it helps someone.