It might be possible, and at least on first consideration sounds like a good idea. I have (briefly) wondered the same myself in the past. It’s one of those small things that rarely matter, but when the need appears it seems so logical.
I won’t be able to look at it properly until after the forthcoming release though, but have made a note for myself to follow up.
I think there should be an easier option than the “+” though. Perhaps simply a new blank field created each time you enter something in the existing field and exit that field. That should be possible with JavaScript.
I wonder if there are other places a similar feature would help?