Hi Parkbremse,
I do know of someone who started his residential ATPL ground school before finishing his ICAO PPL. Of course the ICAO PPL was passed and issued before sitting the exams.
It is perfectly possible (theoretically, I'm not sure about legally) to learn the ATPL subjects required before obtaining an ICAO PPL or indeed any flight training but would be difficult (as if it isn't already!).
What does the first three months or most, if not all, integrated courses cosist of? Yes the first module of the ATPL studies. You'd take the first lot of exams before any flight training.
However if you're going modular and will be studying for your ATPLs via distance learning then I'd not recommend this as having an ICAO PPL will at least give you a little bit of prior knowledge of flying and its subject matter.
Best wishes,
Charlie Zulu.