Apologies for the delay. This feature has now been implemented.

On the "Favourite Servers" page, there is now a green "Add server by IP" button - clicking this will allow you to input an IP address. Please note, the server must already exist in the database (in other words, must be visible to Steam) before you can add a server to the favourites list.

Thank you for your suggestion - we're going to implement this into the website soon.