In Spain there isn't any external cargo endorsement but nevertheless, after some accidents occurred during firefighting, our Civil Aviation Authority came out with new regulations for that type of operations and since year 2000 there is a new rating called "Agroforestal" that qualifies to do those jobs, mostly water bombing which in some sense is external load operations. It's suposed to be about 90 hours of ground training and about 15 hours flight.
Not that this rating teaches you everything about fighting fires, but still a first step towards getting you in the way of learning with some previous advice. I guess it's the same process for all ratings, they don't teach everything but give you the knowledge to start learning, after that you need to get experience and is up to operators to get the right people for the right job.
Buen vuelo
