The Speed actress was previously linked to the project, but denied having any contact with Stallone about the film.
"I don't know where that came from," she told Empire. "I never read the script, I didn't even know what the script was about. But now that I know what it's about, I'm excited for him."
However, Bullock admitted that she would love to return to working on action movies. "Absolutely, but finding a good one is not easy," she said.
Bullock is currently promoting romcom The Proposal, which is released on July 22.
The Expendables will follow the story of a group of mercenaries tasked with overthrowing a South American dictator. Jason Statham, Jet Li, Dolph Lundgren, Charisma Carpenter, Mickey Rourke, Stone Cold Steve Austin and Arnold Schwarzenegger are among the cast.