Currently, when you add a new song to your playlist, it's automatically added to the bottom of your playlist if it's custom ordered. I have a playlist with over 400 songs that I moved from youtube by using a third party tool. Now when I add a new song I have to move it from the bottom to the top manually, which takes a while, because I have to go thru all 400 songs. I know about the sorting by date added, but when I do it, the songs that I ported are reversed, because the porting tool imported them from the first one to the last.
In youtube there is an option called "Add new videos to top of playlist" which you can turn on and off and if you could add the same thing it would be great. As a developer, I see this as not so hard to implement and it would really help me and a lot of other people for sure. It was suggested a while ago, but the idea was closed and probably forgotten.