Thx for your link! After reading most of it I got referred to § 61.75(e)(3), which clearly states that the piggy back FAA lic. bears the same limitations as the foreign license, be they as uncommon for FAA land as they might be.
Regarding the "required training":
If one has "enough" recent flights in the logbook the instructor might as well decide one is immediately "ready for the flight test".
However, in case of an experience/skill history of a typical EASA PPLH w/o night rating (this is a separate voluntary training augmenting the PPL, over here), the instructor most probably would subject the applicant to the whole FAA night VFR training, I'd presume.
So for a holder of an EASA PPLH aquiring a full FAA PPLH would mean at least:
- written test
- training for night VFR (unless one holds an EASA night VFR rating, too)
- flight test
am I missing something?