Yes! To add subdomains you must manage your own DNS settings at your registrar's website. If you are letting Microsoft manage your DNS settings with NS records, you can't add subdomains.
Typically, you can add up to 900 domains to your Office 365 subscription.
For example, you could add the domains contoso.com and contosomarketing.com, and then add the subdomains www.contoso.com, www.partners.contoso.com, www.partners.marketing.contoso.com, and so on.
When you add multiple domains to Office 365, you can host any of the services (like email) on any of the domains you've added. When you change your email to Office 365, by updating a domain's MX record, ALL email sent to that domain will start coming to Office 365.