I know that at least Star Trek: The Next Generation and the Golden Girls accepted freelance scripts. You can sometimes find the submission guidelines for Star Trek: The Next Generation on fan sites. You can also tell if this is the case for a show by looking at the writers. If there are lots of writers who only wrote one or two episodes, the show probably took submissions.

All the old books about television script writing I've read are about submitting in this way, but by the time I was actually considering it episodic television was rapidly dying out.