How is the post_thumbnail information stored?
If you've already created an attachment with the image and are storing the attachment's
post_thumbnail, then you just need to go through your posts and update the meta using
update_post_meta($postId, '_thubmnail_id', $attachmentPostId).
post_thumbnail stores the path to an image file on your server, you need to create an attachment using wp_insert_attachment and pass in that file location. If the files are not already stored in the uploads directory, you can use the Add From Server plugin to import the files into wordpress as attachments. You can then use
update_post_meta to set the attachment as your featured image.
post_thumbnail stories the URI of the image, you need to save it to your server, then create the attachment, then update the post's meta.