It's possible for you to put whatever you want in your 15 slots of the AMCAS application. You could put that you were the lead role in "Humpty Dumpty" in 4th grade. You could put you were in a 4-man kazoo band in middle school. You could put... Well, you get the drift. Essentially, you have 15 spots and you put everything from jobs to scholarships to research to shadowing to volunteering to hobbies. Those 15 slots can be filled however you want.
Now, as for whether it's appropriate, that really depends. If you have one "activity" that lists all your volunteering and you make a quick bullet-point list of all the volunteering you've done, then yeah, maybe tack it on. If you're considering using 1/15 of your spots for some high school volunteering, I'd say you should be worried that you don't have enough stuff that pushes that out of the top 15.