To take a course at another CUNY campus, you need to file an ePermit, which then must be approved by the Registrars of Hunter College and of the college you wish to take the class at. (All of the approvals take place online.) You can learn more about the requirements to qualify for ePermit on the Registrar's Office web page.
To file an ePermit, log into the CUNY Portal and select "ePermit" from the Applications/Resources box on the left side of the screen. For more information about the CUNY Portal, visit this page.