I have never heard of any anti-evolutionist say that you can make hundreds of new DNA bases at random, so I think you are putting words in the mouths of others. I do not hold to fixed species in the sense you speak of either. Species have a very large gene pool. That's how we can get everything from wolves to chihuahuas to great danes from a single gene pool without any mutations.
How did the various species of horsey's all get different chromosome numbers, considering that you obstinantly resist the notion of much more minor genetic changes than that even within species.