One of Keegan's first tasks when he took over at St James' Park was to reject N'Zogbia's attempt to quit the club.
Keegan added: "Charles and his agent wanted to move him out during the transfer window and I wouldn't allow that.
"I said if he did his best until the end of the season and felt the same way, I'd let him go - if he still wanted to go. But it would be on our terms."
N'Zogbia, 21, has been linked with Arsenal and Spurs, but has impressed Keegan with his attitude since recovering from an ankle injury.
The Magpies chief, who added the player to the squad for today's game at Portsmouth, said: "His training in the last week has been the best since I've been here.
"If he can learn to be a team player with his individual skill, he will be a hell of an asset for any club."