I found this plugin and seems it is exactly what I was looking for, a way to add manual links as related posts.
If you are accepting suggestions for new features, let me describe a feature that I think can be very useful.
In my case, I find very useful to add related posts by URL. I was envisioning to be able to search by pasting the URL of the related post in the search field and get the post there. Or better, add a related post by URL instead of searching it in the form. For example, adding a url like http://<domain-name>/post-slug/ and making the plugin detect what is the page.
This may save lot of time for those like me that search the related results by navigating the website, and then want to paste the url only.