Do you have links to the Ecwid products on the main site? Search engines (including Google) will follow (crawl) and should find those links and catalog those pages. Pages don’t have to be on a sitemap for search engines like Google to find them. Sitemaps are just a starting point for the crawlers (bots). Things in the sitemaps may get cataloged a little faster.
If you want you can setup and manage multiple sitemaps.