The hook that makes this possible is
wp_import_existing_post. Strangely enough, it didn’t show up on the Codex. Maybe because the importer is technically a plugin?
You can find this in the wordpress-importer plugin
/** * Filter ID of the existing post corresponding to post currently importing. * * Return 0 to force the post to be imported. Filter the ID to be something else * to override which existing post is mapped to the imported post. * * @see post_exists() * @since 0.6.2 * * @param int $post_exists Post ID, or 0 if post did not exist. * @param array $post The post array to be inserted. */ $post_exists = apply_filters( 'wp_import_existing_post', $post_exists, $post );
Here’s a gist
I pasted this gist from github user ssuess in my functions.php file and then re-ran the import. It worked great!