I have a main site with a bunch of add-on domains that I control under that site in cpanel. Google has already added a sitemap for the main site in the public_html folder. What I'd like to know is, do I need to create a public_html folder under each add-on and include a sitemap for each of those too?
sitemaps are domain/subdomain specific. Subdomains need to have their own sitemap. You might want to create a robots.txt entry for each site that lists the sitemap associated with the domain/subdomain.
in the robots.txt file for each site, you can enter a line that points to the related sitemap. The syntax is sitemap: http://www.example.com/sitemap.xml
you can create a sitemap using web based tools. Enter the URL, let it generate it and upload the sitemap to your website.
All you you have to do is add the Google sitemap in the directory folder of your add on domain. For example I have blog2 as an addon directory (folder)for a second wordpress blog on my main account with its own domain. I just loaded the sitemap in that folder and submited it to Google.