I apologize, I hadn’t seen an exporting option in Lightroom which says “Write keywords as Lightroom hierarchy”. This actually writes the field lr:hierarchicalSubject in the .xmp file as @guille2306 said, e.g.:
lr:hierarchicalSubject
rdf:Bag
rdf:liMexico|Mexico City</rdf:li>
rdf:lishop</rdf:li>
rdf:listreet</rdf:li>
rdf:li~ATTRIBUTE|~Number|group of people</rdf:li>
rdf:li~Food|tortilla</rdf:li>
</rdf:Bag>
</lr:hierarchicalSubject>
So, I should be good to go. Thanks for your help!
giuseppe