Actually you can not drag and drop into the new playlist. I tried. Unless I did something wrong. I tried copying and pasting, and it wouldn’t do that either. I have a playlist that I tried first with one one. Still has one song.
I’m thinking the only way is to creat a new playlist each time you want to add new songs, then delete the old playlist. Time consuming, but possibly the only way. I would do a new one before deleting to make sure, but that’s just me.
Or just have several playlists.
Maybe someone else has a better solution.