Novelizations and writing in other people's franchises is for some people dreamwork which allows them to have considerable incomes, while doing what they love--writing fiction. A great many people feel much better when they are given a set of existing places and characters and are told to work with that. The majority of such people are scriptwriters for existing franchises, but some are novelists. And why not? The best-known examples--R.A. Salvatore; Kevin J Anderson; Brandon Sanderson; and a great many others whose careers really took off when they started writing Star Wars or Conan fiction or the like. Robert Jordan wrote eight Conan novels, before proceeding to do the Wheel of Time...
As Brightdreamer and Jamesritchie have said, you need to have at least one book out which didn't tank, and an agent, whom you've told that you are wide open to any franchise novelization offers.
The other, DIY route would be something like this.
First, you find existing windows of opportunities and make a plan: "In August 2014 they'll be accepting new comic book franchise ideas at this indie publisher, in December 2014 they'll be accepting Firefly fiction for a week, and in January 2015 there's a novelization of SuperMario contest" and the like.
http://www.wizards.com/DnD/Article.aspx?x=dnd/youngreaderssubmissions
http://www.stargatenovels.com/submissions.shtml
http://www.blacklibrary.com/Getting-Started/FAQ-Working-For-Black-Library.html#guidelines
http://www.darkhorse.com/Company/Submissions
Check out also the BBC's criteria to write them a screenplay or a novelization. The great sci-fi comic magazine 2000AD accept ideas for their "future shock" section, where they incubate future comic book writers. There are also other comic book companies which accept story ideas.
...If you poke around, you can find other opportunities to do with videgames, roleplaying games, and tv shows and stuff. You'll end up with a list of a dozen opportunities with specific criteria and a timeline. Once one of those happens--even just as script accepted by the BBC or a rt shostory in a comic book magazine--this should raise your stock considerably. People will pay attention and not dismiss you as yet another crank. One success, no matter how small in itself, will open ten more doors.
One of those doors could be an agent, but if not, you can continue the DIY way.
The next level could include B film companies like The Assylum, and on the other hand--instantly recognizable brand names who are over the hill and churn out 3-4 direct-to-DVD films a year--Steve Segal, Van Damme, Lundgren, etc. Once you have something to show from elsewhere (say a radio play by the BBC, or a Wizards of Dragonforce boardgame novel), and have written the first few very convincing chapters based on a new Segal DVD adventure--you can probably come to some sort of agreement with the film company.
And once you actually have a Segal film or a StarGate or some legit game novelization on your resume--now an agent will far more likely say "yes, of course, a pleasure to work with you!" (although this should happen way back after the first Dragonwhores of Gzbl'k novelization), when you look for representation from a serious agency, and will have a far easier job of finding you increasingly glamorous novelization opportunities to do with today's hits.
Or, you can remain a writer who chronicles the adventures of Segal and Van Damme and the like, and that would be awesome too. Year after year more of the industry will know whom to turn to if they need a fast and quality novelization of Drag me to Hell 4, for example. Just like many people prefer to have an existing framework in which to create their fiction, many people also prefer a stable mid-level gig (in a sense, the vast majority do, hence "the middle class"), instead of struggling at the stratospheric levels where you win it all and lose it all... A question of temperament and preference.