A Crisis...

My feeling is no (but consult a lawyer). If the game creators/owners do not grant you the right to use their characters, settings, and stories, you don't have anything. No agent would take you on, really.

Now, if you do get permission or the rights to do this, an agent would be interested if the game is very popular (did you see the DOOM movie trailers? )

What you've written is called a "derivative work," in the language of copyrights. Generally you are not allowed to publish a derivative work without permission from the owner of the video/RPG world. A video game has the same copyright protection as a novel, so it would be very much like "fan fiction," where you might be able to create something, but cannot publish it.

For more information about the U.S. copyright laws regarding this, visit the U.S. Copyright office's website. Here's a link to the section:

http://www.copyright.gov/help/faq/faq-fairuse.html#permission

This is the beginning of the section dealing with existing copyright permissions. The answer to your specific question is here:

http://www.copyright.gov/help/faq/faq-fairuse.html#change

It's probably not the answer you wanted to hear. Sorry. But you might consider contacting the owner of the copyright again. If it's a popular game, it's possible that derivative novels or screenplays are already in the works. In that case, your work might be an excellent fit. Good luck!