Larry Niven also wrote an episode of the
animated Star Trek series,
Slaver Weapon, in
1973. This episode is based on his short story
The Soft Weapon, and is pretty much a
crossover from his Known Space universe, including by name reference, the
Kzin. There were two other episodes mentioning the Kzin,
Time Trap and
The Infinite Vulcan. These were not written by Niven though, they just make use of his character species.
References
http://sflovers.rutgers.edu/Reference/tv/EpisodeGuides/star-trek-animated.guide
http://www.maxnm.com/niven/4.html
gopher://wiretap.spies.com:70/00/Library/Media/Trek/startrek.kzn