@KoiwiGal - Actually I think it gets to the point where some writers couldn't write in a different style if they had to.
I know at one point Stephen King was writing under an alias, trying to see whether he could get published again, or if it was all a fluke. He published several titles under this pen name, never revealing to anyone that it was actually him doing it.
It wasn't until a librarian realized that the writing styles of both authors were so similar that he got caught out. I believe the librarian thought the pen name author might have been plagiarizing, the styles were so close.
And I have been to several author signings and lectures and they almost always sound exactly like they do in the book. The same with poets. Once you hear the voice, its that voice you'll be hearing whenever you read the book.
I don't think anyone can help their writing style.