I'm not sure you can learn brand new things in a dream, but you can certainly practice and get better at something you already know from waking life.

I have read about a golf player that was perfecting his swing in LDs, about a surgeon who practiced surgeries during LDs. And some artists and scientists, that got inspired by dreams and who actually invented things in dreams - like the periodic table.

But first you have to know things from waking life, in order for your mind to help you figure things out.