This question is going to suck. I have an idea to turn a computer game (I feel so cheap) into a screenplay. Just as a side-note, it’s more of the atmosphere of the game that’s appealing, not the quick buck mentality. Obviously I can’t afford to buy the rights, but where do I lie submitting it to agencies? Would they even look at it?
Your question doesn’t suck. I adapted a computer game into a movie ("American McGee’s Alice"), and I didn’t feel a wee bit cheap.
But then, it was a damn cool game, and I got paid. So there you have it.
Since you don’t think you can get the rights to the game, ask yourself if you’re still willing to write a script that can never get made (or even sold). If the answer is yes, go for it.
Some agents will avoid reading your script, simply because they know they can never sell it. But some agents will read it as a writing sample, and if the atmosphere of the game world is as cool as you say, maybe they’ll like it and see you have potential. Make sure you identify your source material on the cover page, which would read:
written by Chris Somebody
based on the videogame "Underpants Wars"
published by Powerful Software Company
Considering you’re probably broke, and the fact you won’t be able to sell this anyway, there’s very little danger Powerful Software Company would bother coming after you. But you’re morally and karmically better off giving credit where it’s due.