If you use Google webmasters tool to have pages in a sitemap from other domains, you can do that. A single Sitemap can contain pages from multiple domains as long as you have verified ownership of all sites.
Do you know that a single Sitemap can contain pages from multiple domains?
About Marshall
Marshall is creative head at Elicus and works with the team to bring ideas to life. He is strongly focused on delivering a quality experience to clients and customers.