It's totally plausible within the Star Wars universe for a non-Jedi/Sith to use a lightsaber. Finn used one. Pre Vizsla used a darksaber. General Grievous used up to 4 lightsabers at once. None of these characters are considered to be force sensitive. They were just highly trained warriors.
In the old Star Wars RPG, anyone could use a lightsaber, but obvioulsy Jedi were much more effective. Also, it took being force sensitive and trained to make your lightsaber block another lightsaber...otherwise it just passed straight through...