This is how you can add an extra e-mail address to an Exchange Mailbox User using a little Powershell script. The script below uses two ways of adding an address to a mailbox.
[sourcecode lang="powershell"]
$Temp = Get-Mailbox -Identity
$Temp.EmailAddresses.Add(‘smtp:john@secondaddress.contoso.com’)
$Temp.EmailAddresses += (‘smtp:john@thirdaddress.contoso.com’)
Set-Mailbox -Instance $Temp

[/sourcecode]
Check out the full procedure in this Technet article.