Add alternative UPN suffix in Active Directory
I'm currently working on converting "In cloud" Office 365 accounts to Active Directory synced accounts. As a step in the process, I had to match the UPN suffix to match the "Username" or SMTP Value in Office 365.
Open the Active Directory Domains and Trusts snap-in.
In the left pane, right-click Active Directory Domains and Trusts and select Properties.
Under Alternate UPN suffixes, type the name of the suffix you want to add.
Click Add and OK.