Short answer: I don't think there's one standard route to becoming a media tie-in writer. Like most careers, the hard part is getting your foot in the door, then one job leads to another until, hopefully, you build up a nice resume and network of connections.
In my case, I had already been working as a writer/editor in SF publishing for a few years before I started getting offered tie-in work: first a couple of BATMAN stories for DC, then a DS9 novel, a couple of IRON MAN novels for Marvel, etc. While also working full-time as an SF editor for Tor Books.
Eventually I segued from being a full-time editor who wrote tie-ins on the side to becoming a full-time writer who still did some book editing on the side.
In the case of ROSWELL, let it be noted that the Roswell books were produced by the same publisher who was doing the Star Trek books, so I was already on their radar. One job leading to another . . . .
Hope that makes sense.
I should also mention that such projects are almost never initiated by the writer. They typically begin with a publisher acquiring a license, then recruiting writers for the project.
"Hey, Greg, we just acquired the ALIAS license. What's your schedule like?"
As opposed to me applying to write an ALIAS novel.
In general, you want to make a name for yourself as a writer, then let it be know that you're interested in tie-in work.
Is OUaT friendly to viewers who may not be knowledgeable to the bulk of what the show is based on? I've never been one for fairytales, even as a kid. The show looks good, but I fear it would be constantly leaving me in the dark. 
