If you can imagine it, you can do it. Of course there are limits, if you were trapped in a white room your entire life, and somehow learned to lucid dream, there would be no way to learn about playing a guitar (unless you managed to invent one in your dream first). If you think you can, you can.
So you’re saying you can only learn something in a dream if you know it exists in real life? Sounds probable to me.
You’re right! 
No exodus, you can learn something in a dream if you can imagine it, because you could learn about something that doesn’t exist in real life.