Whenever you connect you a domain to Office 365, new users will get the email addresses derived from the new domain, but existing users do not.
The next script adds 2 email addresses to existing users. E.g. when you add contoso.com it will add firstname.name@contoso.com and alias@contoso.com to the existing users.
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange –ConnectionUri
https://ps.outlook.com/powershell/ -Credential $LiveCred
-Authentication Basic -AllowRedirection
Set-ExecutionPolicy RemoteSigned
Import-PSSession $Session
$users = Get-Mailbox
foreach ($a in $users)
{
$a.emailaddresses.Add("$($a.alias)@newdomain.com")
$myEmail = $a.firstname + $a.lastname
$myEmail = $myEmail.replace(' ','')
$a.emailaddresses.Add("$myEmail@newdomain.com")
}
$users | %{Set-Mailbox $_.Identity -EmailAddresses $_.EmailAddresses}