You can register with CIMA at any time and with any background. There is no requirement to be working within management accountancy when studying CIMA. However to call yourself a Chartered Global Management Accountant (CGMA), you will need to demonstrate a minimum of three years' relevant practical experience.
CIMA students and members have a wide range of jobs available to them, most of them, but not all of them, within Finance or Accountancy. For an idea of the job roles that CIMA students and members undertake please visit CIMA My Jobs which is our dedicated careers portal for management accountants.