You can take a subset of the ATPL exams for the CPL, and I'm sure I have seen course for these just these CPL exams advertised recently in Pilot magazine.
However I think that very few people do just the CPL exams as the IR exams are also subset of of the ATPL exams but the CPL and IR exam sets overlap so much that by studying only a few more exams you have passed all the exams for a CPL/IR or Frozen ATPL, so you might as well do all 14 exams.
I would imagine that any hiring school would like to see either an IMC or IR rating before hiring you as a PPL(A) FI(R), if only so you can get the student home if you are caught out by the weather.
Good luck with your aspirations, I'm also set on being Flying Instructor when I have enough experience to be able to teach and fly at the same time! (BTW Respect to Aeros Instructors who can teach while upside down!!!)