If you have experience writing policies, share that now. If you do not, tell the interviewer about any experience you have writing formal documents such as research papers. If you have no experience writing formal documents, let the interviewer know you are more than willing to learn the process.

"While I've never written a formal policy from scratch, my current boss does rely on me as a proofreader for the policies she writes. I've reviewed at least twenty new policies for her in the past couple of years and feel I have a strong understanding of the structure necessary to write one. I realize each company has its own templates and preferences though, and I know a brief review of your existing policies would be all I'd need to learn what you were looking for in a new policy."