Question
Is it possible to add and use another .onmicrosoft subdomain under the existent Office365 tenant?
Answer
Subscriber can manage the only one .onmicrosoft subdomain in scope of Office365 tenant.
It is possible to add another .onmicrosoft subdomain on both OA and MOP side, but it is impossible to complete the verification procedure.
Such subdomains always have Not verified status without an opportunity to create users or services, assigned to the subdomain.
This limitation is on the Microsoft side. Please, refer to the "About your initial onmicrosoft.com domain in Office 365" documentation article for detailed description.