Indexing – Can we safely add pages to our Sitemap that Google has already indexed?

you absolutely should Add any pages already indexed to the sitemap, otherwise your sitemap will be incomplete. That's not negative, only positive.

A sitemap should Always contains the full list of URLs you want to see in the search, Period. Whether the URLs are already indexed in search engines, is irrelevant.

Do not forget that Google is the biggest player, but not the one just Player. Many small search engines and other web crawlers benefit from a complete and accurate sitemap.