It seems the answer to that question is, “by some good old-fashioned networking and co-operation”.
According to Variety,
“A recent example of the value of co-agenting can be found in UTA’s brokering with the Writers House a Summit film deal for “New Moon” and “Eclipse,” the second and third installments in Stephenie Meyer‘s “Twilight” series, with UTA screenwriter client Melissa Rosenberg also in the mix.”
What’s also notable about the article is that it only mentions the first three films. Numerous actors have recently been quoted about wanting to do Breaking Dawn, that the fans seem up for Breaking Dawn, etc.
Here’s our specualtion( we have no insider info, this is pure guesswork here based on information patterns): While people may want to do Breaking Dawn ( I mean who doesn’t want more money on a sure-fire blockbuster chain?) it doesn’t seem like Breaking Dawn was part of the initial deal. If it is in negociations, it doesn’t look like ink is dry on that contract. If it were, Summit surely would have announced that, and Variety would be talking about it in this article.
Now, if the Screen Actors Guild could just get their contracts settled…