As you have already done the EASA ATPL theory, it seems you wish to fly commercially in the EU.
Why not do the EASA CPL first, which you can do with a FAA PPL and ATPL theory and then bolt the EASA IR onto the CPL. You could in fact do the two courses in tandem, CPL then IR then apply for the CPL and IR at the same time, saving you admin costs as well.
This will save you having to convert the FAA PPL to an EASA PPL.
If you do want to get the EASA PPL, you will not have to sit any PPL exams as any theory exams passed at a higher level are valid for licence issue at any lower level, i.e. ATPL exams are valid for CPL and for PPL.
Hope that helps.