Known to, but not used by. You could be right that there were men who used magic, but none in this story.
No Gandalf, the rest of the Istari or Sauron are not men, but the Ringwraiths were. They were Kings and Queens who were also powerful sorcerors.
I don't remember the reference to the Nazgul being sorcerers. Were they sorcerers before or after they ceased being men? I'll comment a little more about them later.
The Lord of the Nazgul is called "The Witch King" and the Mouth of Sauron was a man who was a powerful sorceror. So yes, men did wield magic.
And now to my other point about the Nazgul. Does Middle Earth contain stories of any men who use magic who remain its master? Or does the magic always overpower the man? That's a real question for you, since you seem to have read some of the other things written by Tolkein. I've only read The Hobbit and LOTR.
Shalom.