When I applied to the Master of Fine Arts in Creative Writing program at UNC-Wilmington, I took along some of my published work to illustrate that I am a professional writer. The interviewer glanced briefly at my Star Trek and Doctor Who and MechWarrior titles, then suggested I might want to try a few undergraduate writing courses first to discover whether I was really ready to write seriously.
The prejudice against media tie-in writing is particularly virulent in the halls of academia -- but they tend to view any fiction that sells to a commercial market with suspicion, if not alarm. However it can be found almost anywhere writers and critics congregate. Media tie-in writing is write-for-hire. You are writing a story set in a universe designed -- and owned -- by someone else. Where a writer of original fiction is completely self-directed, a media tie-in writer is part of an ensemble. One is a soloist, the other first chair with the Philharmonic. Different disciplines applied to the same art.
It's a different sort of business as well. In original fiction, you write a story and then you sell it. In media tie-in writing, you sell a story and then you write it. You must first pitch your novel idea to the editor. A pitch is one or two sentences that capture the high concept of the story. If the editor likes the pitch, she asks for a proposal (though some call it a treatment), which is a summary laying out the bones of how you intend to tell the tale. Some editors like short proposals, one to three thousand words, while others like something more in depth. The editor then takes the proposal to the licensors, the owners of the intellectual property, and if they like it, you get the go-ahead to write.
(At this point the process is similar to screenwriting. In an article about "Sister Act" appearing in this week's New Yorker, Paul Rudnick tells us "I wrote a treatment, which is a studio term for: Summarize the story in less than two pages so that an executive assistant can boil it down to one paragraph on a Post-It.")
With the go-ahead comes a contract, usually one-third on signing, one third on delivery and one third on final acceptance of the revised ms. Typical deadline for a full-length novel is 120 days.
Breaking into media tie-in writing requires research and persistence. You have to know who to send your pitches to, for one thing, and you have to know the market well enough to be able to deliver what they are looking for. Case in point: I have been pitching to Black Library for three years. A few of my projects have gotten to the proposal stage, but nothing has connected. As one editor put it, I'm too damn cheerful. Of the dozens of Doctor Who short story pitches I sent to Big Finish, only one connected ("The secret of the golem is slime mold and cockroaches. The Fifth Doctor and Turlough must solve a mystery and confront an enemy hundreds of kilometers beneath Prague to save the city from annihilation.") and before that one was approved, I had to change the companion.
Novels are not handed out to unproven writers, of course; you need to establish a track record. Which means any short fiction market related to the IP you're interested in is the place to start. With short fiction there is almost never a proposal, with the project going straight from pitch to story. A few markets take unsolicited stories. (Which ones? That's where the research comes in.) Do not start with fanfic. There are some excellent fanfic writers out there, but fanfic borders on copyright infringement and you do not want to be mistaken for a pirate.
Finally, you have to understand that nothing you write for an intellectual property you do not own is yours. You write the words, you sell them, you have no rights to the story. I had to get the IP owner's permission to post one of my BattleTech short stories in my Live Journal. (Link to story: "What I Remember Most")
If you're unbothered by not owning the words you write and think you'd enjoy the challenge of creating something that is both uniquely yours and part of a greater work, you may want to look into media tie-in writing. I've loved it for nearly a decade. It sure beats writing seriously.