WordPress is set up in a way that mostly ignores existing files. If what you want is literally upload fixed sitemap.xml file you can just do that and upload it to the root of your site (using FTP, hosting control panel, or whatever). WordPress will just ignore it and it will be available at example.com/sitemap.xml as anything else would be.
Of course it won’t update dynamically with your WP site. To do that without a plugin you would have to essentially write a plugin that does it yourself.