Rich,
Before getting a CPL, you must have completed either the CPL exams or the ATPL exams. Before getting an IR, you must have completed either the IR exams or the ATPL exams.
So, to do what you're suggesting, you'd need to do both the CPL exams and the IR exams. There is considerable overlap between the two, and you'd end up doing more work than is required for the ATPL exams. Then, afterwards, you'd have to do the ATPL exams, which would effectively be duplicating all the exams you've done already.
So yes, it is possible, but it would be a completely impractical, if not totally ridiculous, way of doing things.
FFF
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