Can you name a language pair where that works? I can’t think of one.

If your native language is Ukrainian, when you learn Russian you don't get more phonemes, unless you count slightly different mouth positions of vowels.

Oh, right. I forgot all those Dutch speed skaters who used to train in Norway a couple of generations back. Ard Schenk.. Kees Verkerk and many others. The majority of them learned Norwegian completely accent-free (you really had to listen to figure out they weren't native), despite not learning the language as kids. I figured that must have been because Dutch has such a variety of native sounds that it basically covers Norwegian language sounds.