Thank you for your answers.
I booked it directly through Cunard and there it was not possible to book a transfer to Paris for Le Havre.
There is, however, an excursion from Le Havre to Paris via Cunard ("Paris Entdeckungsreise), but this is probably only for guests who are travelling on to New York and not for guests who want to stay in Paris with their luggage..
One person on this trip only has a visa for Germany and France, but not for the UK, so I'm unsure if this visa also needs to be available (Even if there are no plans to go ashore, some countries require a transit visa.)