Grace, 27, who played Shannon Rutherford during the first two seasons of Lost, has signed on to play Irina, a member of the Denali coven who blames the Cullens for the death of her lover, Deadline.com reports.
Meanwhile, Foy, a 9-year-old actress with no feature-film experience, is close to taking the role of Renesmee, Edward and Bella's vampire/human love child, according to EW.com.
Breaking Dawn, the fourth and final novel in Stephenie Meyer's Twilight series, is being split into two films, the first of which will hit theaters on Nov. 18, 2011.
Screenwriter Melissa Rosenberg and director Bill Condon were still working on script rewrites earlier this month. The first film goes into production this November.