Most of my website pages are indexed in Google. But in my website's XML sitemap we have fewer pages than the number indexed.
Can we add all those indexed pages in my sitemap.xml file? It hurt anything?
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The sitemap does not affect rankings, it helps crawl-ability of your site. Is common for Google to index all pages found on your site even if they aren't on the sitemap.
On short: Having a sitemap is not a ranking factor. It just helps Googlebot discover page which otherwise couldn't. It also helps Google index pages faster.
You absolutely should add all those already-indexed pages to the sitemap, otherwise your sitemap is incomplete. There are no negatives to doing this, only positives.
A sitemap should always contain the full list of URLs that you want to show up in search, period. Whether or not the URLs are already indexed in any search engines is irrelevant.
Don't forget that while Google is the biggest player they are not the only player. A lot of minor search engines and other web crawlers will benefit from a complete and accurate sitemap.