The ensemble cast lining up now includes Greg Kinnear, Diane Kruger, Eva Mendes, John Malkovich, Peter Sarsgaard, Peter Dinklage and Zooey Deschanel. All are in talks but no deals are confirmed yet.
The project is based on a bestseller by Christopher Buckley, who also wrote hit Thank You For Smoking.
Omega's Peter Rogers signed the deal with producers Edward R Pressman and Brett Walsh (Glasshouse Pictures).
The satirical comedy is about the US government staging phoney alien abductions.
The film marks Stillman's first return to directing after 1998's The Last Days Of Disco.
Pressman also executive produced Thank You For Smoking and he will again produce another Buckley adaptation, God Is My Broker.
Omega's slate also includes Heartless starring Jim Sturgess, Grimm with Bruce Willis, The Six Wives Of Henry Lefay, comic book adaptation Mandrake and epic drama Purple Mountain.