You are still able to fly as an F/O for an airline without a ATPL. Many F/O's has CPL/ME/IR and a so called frozen ATPL (only written the theoretical exams). ATPL is only needed when taking the step to command in the left seat, so when time is ready and the company wants the F/O to step up the pilot takes the checkride (as long as he/she has the hours/age).