Dictum. Living your life as if X is true and believing that X is true are one and the same.
So let's consider the atheist's question, but rephrased. Can a current atheist will to choose in the future to live in such a way as if atheism is false? If they can, then by our dictum, they can choose to believe in God. By the way, this isn't just true for theism, but for anything. If you live your life as if X is true, then that is the same as believing that X is true.
Here's a quiz you should pose to anybody who doubts that they can choose what they believe?
Question #1: Can you choose to give away all your money to a town drunk?
Question #2: Is it an easy choice for a person who worked hard all of their lives to earn their wealth to say 'yes'?
Question #3: If you've answered "no" to Question #2, then does that logically imply that the answer to Question #1 also be "no"?
Question #4: For any X, can you choose to plan your life and behave as if X is true?
Question #5: Is there any difference between choosing to plan your life and behave as if X is true and believing that X is true?