put a "Volunteer Experience" section below your job experience and add it there. Depends on how much you have on your resume. If you have a lot of work experience but not all of it is relatable to the job, I'd do a relevant experience section and include the volunteer under there. If all of your experience is relevant to the job, do a Volunteer section.
This. I have this section on mine, including all of the hours that I did with APO in college, and some of the special projects that I worked on.