[Auridan]

As usual, I'm going to tell you what my own research has uncovered, and someone who knows better will correct me if I'm wrong.

The specialties you're looking at are called NECs, Navy Enlisted Classifications. These are usually advanced training that comes from a C school, and you're not likely to get it in your contract. I assume you land one by expressing an interest, and then being a flaming badass in A school, to maximize your chances of being sent to a C school at all. But HM (like so many things) is pretty overmanned, so I wouldn't count on anything.