Several of the Relaunch novelverse TrekLit authors tried out with spec scripts before being picked up to write tie-in fiction.
David Mack, a film school grad, got script credits for 2 DS9 episodes, Starship Down and Only a Paper Moon before being contracted for some Starfleet Core of Engineers stories.
Kirsten Beyer, a theatre grad, never got into one of the shows with a spec script, but was picked up to write Voyager books, then came full circle to be in the writers rooms on all the new live-action shows.