Of course you can put freshwater fish in salted water!
You just have to remember to add black pepper and veggies, otherwise the soup won't taste as good.
EDIT: On another note, a scientist from the Philippines have developed a system where he puts fish to "sleep", takes it out of the water for hours, then puts it back in water to revive the fish. This one's for real, he won an award for it. They'll be using this technology lessen shipping costs of fish (no water needed to ship fish ergo lighter cargo load = cheap fresh fish). I'll post the link when I find it.
So I guess not all incredulous news reports are fake.
Found it:
Sleeping fish