Playing the first two games is never a bad idea, as they will, indeed, provide some useful background, but it's not strictly necessary, in order to enjoy Wild Hunt, and the expansions. You can dive into Witcher III without the previous games, and visit them later, if you like.
It is possible, however you would miss some parts of the plot. Learn more about the universe - I always recommend it, I recommend books, they are awesome. The game is absolutely playable and entertaining even it this is you first experience with this universe. But you would miss a lot of very interesting content which would help you to better understand what's going on and why.