Doesn't change my answer. If you haven't done it yet you can't list it as a current activity. I can tell you're eager to have this activity on your application. The only way you could honestly include it now would be to list it as "X Activity, starting (date), anticipated hours". Frankly, I'm not sure it's worth it. Despite what you'll hear here there is no requirement that applicants to schools have community service hours. What admissions folks want to see is a track record of meaningful activities, whether those be paid work, volunteering, sports, music, clubs or any other extracurriculars.
Your new volunteer job might alternatively be put in the "additional information" area. You could explain that as of this date you have not yet begun volunteering but that you have made a major commitment to future hours at the hospital. Again, I'm not sure it's worth it, but it's up to you.