Rewriting .htaccess rules for sitemaps in multisite with domain mapping

I've been having issues with Smartcrawl and the sitemaps of the different sites. Apparently, there was a CORS issue, which is now resolved. So, I can find the sitemaps in /wp-content/uploads/sites/2/sitemap.xml and they work. 
The only problem left is that I need to use htaccess to create the appropriate rewrite rules.