Good question. Yes, that was Niven adding his space cats to Star Trek.
He adapted one of his 1960s Known Space stories, "The Soft Weapon" into a Star Trek animated episode called "The Slaver Weapon". You can find The Soft Weapon in a collection of Known Space short stories called "Neutron Star".
"Canonical" Star Trek has avoided referencing the Kzinti again, but they remain a favorite of various elements of the fandom. Kzinti are also one of the major factions of the "Starfleet Battles" series of games, which has evolved separately over the years from Star Trek's ordinary continuity. As was the style at the time, Alan Dean Foster adapted all the TAS episodes into novels called 'Star Trek Log {number}'... so, in Star Trek Log Ten you can read Foster's prose adaptation of Niven's script adaptation of Niven's original short story!
Niven also wrote some of the daily 'Star Trek' comic strips in the 1970s, and these may also have included Kzinti at some point (I don't honestly remember).
Also -- TAS will finally be out on DVD next week, the last Star Trek show that needed a release... so if you want to see cartoon Kzinti again, now is your chance!
The actual "Man Kzin" wars books are primarily spinoff stories written by other authors, and they're far more recent than most of Known Space -- the Man-Kzin series started some fifteen years ago, around the same time the original Wing Commander came out. The Kzinti themselves were created (I *think*) in 'The Warriors', a 1966 short story. They've since been background players throughout Known Space, most famously in the Ringworld books.