Can you do roleplaying writing for someone other than an RPG company? Sure.
There are a small handful of RPG magazines (and a large handful of web-zines) out there. Some of them pay a wee bit, most of them don’t pay at all, and one or two pay a surprisingly high amount. Some of them will take almost anything, some will take very specific material that you will find described in their writers’ guidelines, and one or two prefer to use in-house people and will heavily re-write things by outside authors. In other words, there’s a wide range.
One way to give yourself that first small start in the RPG-writing field is to write for the large number of RPG-related web sites, fanzines, etc. out there. One of the stated purposes of RPGNet’s vast reviews section is to give people a chance to put out a little RPG-related writing; to perfect their craft before putting it before developers. The RPG Times has been similarly eager to give writers a chance (in past experience). You can submit to other web-zines for the same reason. If you put together a small portfolio of articles, you can use it as sample material when approaching a developer, especially if several of your articles relate to his game.
Just keep in mind that you’re likely to make even less doing this than you’ll make writing RPGs themselves. (In fact, odds are you won’t make much at all.) Also remember that just because you aren’t getting paid much is no reason not to do your best work. You don’t want a potential employer to see your half-assed article on some web site and remember it later on when you’re trying to get hired.
Writing for RPGs vs. Horror Markets
I have some experience with the horror market, and I find it to be quite different from writing for the RPG industry. For RPGs, I get outlines and write what other people tell me to write for a specific deadline. For the horror market I write what I want to write when I want to write it, and then submit it to magazines and anthologies (often shopping a story around for a while before it finds a home). It’s a very different process. Some people honestly aren’t suited to both. Some people need direction, and aren’t very good at writing their own stuff off-the-cuff. Others are wonderful at writing their own stuff and can’t take direction to save their lives.
The best favor you can do yourself is to figure out which you are. If you’ve only done one, then practice the other a bit. There’s no sense giving up on something you’ve only done a couple of times, and many people are perfectly capable of doing both types of writing. But if it turns out that you just can’t stick to an outline even for a million dollars, then do yourself and your developer a favor and go back to writing your own material.