The difference between CPL and ATPL is not that much. The syllabus is slightly larger but the standard of the questions asked is about the same.
If you have ATPL exams, then you don't need to sit CPL exams. However, if you sat CPL exams and later needed ATPL exams, then you would have to sit the whole lot.
You might as well do the ATPL exams.
Cheers
Whirls