|Sifu Kisu, in all his epic epicness|
Here's the source, which confirms a lot of other workers too.
Sifu Kisu Back in Action for Legend of Korra, Plus Others
The article also says that Dee Bradley Baker is confirmed as a voice actor on Korra. If you haven't memorized the credits, he is the voice of Appa, Momo and all the other adorable creatures on Avatar. Bosco anyone?
On a Side Note
This isn't related to the article, or even Legend of Korra. But I just wanted to announce that soon, I'll be putting up articles looking back at the original Avatar series. I've been re-watching them in preparation for Legend of Korra. So look forward to that (or not) coming soon!