I get where the confusion comes from — names in the galaxy far, far away tend to blur together — but no, 'Kanan Stark' is not a character in the official continuity. When people say Kanan they almost always mean Kanan Jarrus (real name Caleb Dume), who is absolutely canon thanks to the TV show '
Star Wars Rebels'. When people say Stark or Starkiller they’re usually thinking of
galen marek from 'The Force Unleashed', which lives in the Legends continuity now. Putting the two together as 'Kanan Stark' reads like a mash-up or a fan-made original character more than something Lucasfilm ever produced.
If you want to be picky about terms: after 2014 the Lucasfilm Story Group tightened up what counts as canon. Movies, the live-action and animated shows, and most tie-in books, comics, and some games released under that umbrella are canon. Lots of older Expanded Universe stuff became Legends and can include characters like Starkiller. Fan creations, roleplay OCs, and crossover nicknames frequently spawn hybrid names like 'Kanan Stark', and they can be fun — just not official.
I check Wookieepedia when I’m unsure; it has separate entries and labels for Canon vs Legends, and that usually clears things up fast. Bottom line — enjoy the fan mashups if you like them, but don't look for 'Kanan Stark' in the official continuity. Personally I love seeing creative blends, though; they say a lot about how much people care about the universe.