If you build up your rep enough in Brazil with a strong record of research, you will probably have a better shot than just joining a society. Programs that are willing to look at foreign trained physicians typically have a heavy emphasis on research.
Also, if you are looking to become a board certified dermatologist here in the states, you'd have to do your residency here (except in very rare circumstances), not just a fellowship.
If you are looking to stay in Brazil and just want the fellowship to state that you are an American trained physician to gain prestige, then it may be to your benefit to do an internal medicine residency in the U.S. and then do your dermatology (or other advanced training) in Brazil. I know several people who have done this route (mostly in other fields, though) and have gone back to their home countries and made $$$