You can use the Facebook comments abilities in addition to other comment extensions, like JomComment. However, comments posted into either will not show up in the other.. so it will actually be 2 different comment areas. One for your 'existing' extension, and another comment box/comments area for Facebook. Probably not what you're looking for, but might work for you.
As for Kunena, yes, we integrate with it. I'm not sure exactly what integration you're looking for, though, so I'll explain what our integration does. Right now, when a user creates an account using Facebook, their Facebook avatar and profile is imported into Kunena. The profile fields are completely configurable, so you can take the user's interests or bio and use it in their Kunena signature, or any other Facebook <-> Kunena fields can be easily configured. Additionally, when the user logs in, you can have JFBConnect update their Kunena profile/avatar if it's changed in Facebook.
Hope that helps explain. As always, we have a 30-day money back guarantee if you'd like to try JFBConnect out and see if it will work for you!