>So, does this mean, that （何か）使い is basically a '(Something)-user'?
That's true. But at least it should be a noun.
By the way, when I watched Hunter x Hunter with English sub, I found the translator gave "Nen-user" for the translation of both 念使い and 念能力者. 念能力者 would be , if literally translated, "a person with Nen ability".