User blog comment:Chichiryuushintei/High School DxD BorN Episode 5 Review/@comment-26285797-20150505133335/@comment-26322543-20150506031632

That's because their Volumes were mixed into one arc. V6 won't have anything major from other Volumes and was said to be adapted similarly to Season 1&2. So such a big change wouldn't make sense.

Remember, there are 3 whole episodes after Volume 6 is done, the curse and more development with Rias will probably be put there.

Not saying it's impossible, just that by the information we got, it's unlikely.