What happens when legal GEDCOM data that is not supported by FO is imported?
FO will generate an error in the .lst record and ignore the data. It doesn’t matter whether it is legal 5.5 data or not. It only matters if FO can understand it. For this reason I would recommend a trial import of any foreign data into an empty database before adding it to your live data. This way if you need to hand edit the GEDCOM file to get the data you can do that and re-import. You only get one shot if you are going to your real database.