In addition to Cargoship Cruises provided by @andra there is another website that provides similar information Freighter Cruises operated by Maris which upon close examination has a Cruise on a CGM CMA Freighter between Europe and South America(see Blue Route). If you look at the list of stops one of them is in Cayenne, French Guiana, which is what you're looking for.
One thing to mention. If money is no object you can try to rent a Yacht and cross Atlantic in Style.