Writing Your Own Job Description: Good Idea?

Is writing your own job description a good idea?

Here’s the context, from a ATH reader:

I came in second — didn’t get the job. But the company interviewed me again for a unique new position based on my unusual set of skills. They want to create a job for me, but prior to interviewing with the company president, I’ve been asked to write up the job description. I think this is genuine, but my research on the president indicates some issues.