[cflsystems]

You do not have to add anything. It is already there. And it will sort by position. But you need to set this position in admin - see my previous post.

Now if you are talking about allowing customers to sort by position - that's something else. But I don't really understand why customers will care about this. The position field is more of an internal thing than public.
Anyway you can still do that but you need to make changes to how products show and the sort by dropdown filter. The field I believe is called position but check ./Model/Product.php to be sure.