Probably it is not the best solution but it is compact enough, it is what I ended up writing:

    public FilteredDE map(Result re) {
        FilteredDE filteredDE = new FilteredDE();
        for (Group group : re.getGroup()) {
            for (Record record : block.getRecord()) {
                map(record, filteredDE);
            }
        }
        return filteredDE;
    }

    @BeanMapping(
            nullValuePropertyMappingStrategy= NullValuePropertyMappingStrategy.IGNORE,
            nullValueCheckStrategy=NullValueCheckStrategy.ALWAYS)
    public abstract FilteredDE map(Record re, @MappingTarget FilteredDE filt);

Is there any better please?