The ancient language is just the mechanism for invoking magic and just because it's ancient doesn't mean it's complete and set in stone. Magic appears to be its own entity that permeates all of reality and changes and evolves right along with the world. So it's not far fetched to say the language also changes and evolves. Examples of this change are even referenced by the fact that a person's true name can change over time.
I always thought that learning a word previously unknown or forgotten took a similar process to how one learns their own true name. It comes about through intense study and understanding of the innate nature of whatever you are trying to name in the ancient language. Once you completely understand something the word just sort of comes to you by instinct and "feels right". The ultimate authority is whether or not magic itself/reality responds to the word. So yes if you think about it magic kind of comes of as having some form of sentience and that "right feeling" you get is essentially you feeling the magic responding to your complete understanding.
There are several instances of learning a word/name I recall from the book that come from the result of intense study
1) As mentioned in the other answer Eragon learns the name of the sword Brisingr. Since he was involved in every aspect of it's creation it's only natural he'd have a complete understanding of it.
2) Eragon also figures out the true name of Sloan while reflecting on him and his motivations for being the way he is. This came in part because Sloan is a very simple and straight forward person at his core so he was easy to understand once you got down to it.
3) All the separate instances of people in the story learning their own true names, there are many. Eragon learning his own true name is obviously the most detailed example.
I would also wager that you could use the name of the ancient language itself to create a new word but I don't remember any overt examples of this.