The Girls Aloud singer will star alongside comic Hugh Dennis, Extras actress Ashley Jensen and rapper Ashley Walters in the movie.
"I am thrilled to be joining the cast of All Stars," she reportedly said. "I have worked with Theo before on Horrid Henry and he is a huge talent.
"I loved the script as soon as I read it. It is a heart-warming family film that will put a huge smile on your face."
Producers Allan Niblo and James Richardson reportedly added: "We are delighted to have Kimberley for the role of Trish, she is a brilliant actress with an electric screen presence. She joins an amazing cast in what will be an exciting film."
Walsh recently finished her sell-out Shrek the Musical tour as Princess Fiona. Kimberley is also expected to reunite with Girl's Aloud bandmates the group's tenth anniversary later this year.