I direct adult A Cappella community type choirs, and my experience is that no one who's ever come to a choir who can't pitch a given note, has ever learnt to do so. They might get some of them, but it's hit and miss!

The potential singer gives up, even after a few individual singing lessons. Most don't even get as far as singing lessons.
Not from me I might add. I wouldn't have the patience!

So it may be possible, but I've yet to see it.