Hilchos Lashon Hara, chapter 2, section 9

Even when it is allowed to repeat lashon hara that was said in front of three, this only means that the repeating itself is not forbidden.

But it is forbidden, Heaven forbid, to add even one word, or to make the story sound more believable to the listener. That would cause more harm than if the story had just spread by itself (which is the reason "in front of three" is allowed). Also, making the story sound believable shows that the person accepts it as true.