I studied this in biology. Sometimes RNA is replicated incorrectly and adds something else. For example : GCACCAG would turn into GCACGCAG because a C got in there.
Mutations can add or subtract information, as permitted by DNA. That is, we can't possibly grow a dog's ear, because our DNA would never allow it, even with mutation. It wouldn't be possible for horses to have eagles wings. I'm sorry for not being clear, but THIS is what I meant when I said mutations do not add information.