Sorry, there is no such tool available. This is not easy to do if the website where you import already has posted. New, imported posts, have to have new ID’s, update all meta, tags, users and on top of all that, all rating, data needs to be updated too. I am not aware of any commercially available backup plugins that can import and merge posts, let alone do it with ratings.
This would most likely require custom-built merge solution. The connection between ratings and posts (and other rating items data), is in the table ‘wp_gdrts_items’ and column ‘id’ in that table is ‘ID’ in the wp_posts table (when the ‘entity’ column is ‘posts’).