It would probably be easier to use Set-MailPublicFolder to set the entire collection of email addresses that you want. E.g., create an array with the existing one(s) as well as your new one, and then pass that to -EmailAddresses. But you also need to provide an email type identifier, as described at https://technet.microsoft.com/en-us/library/bb123707(v=exchg.160).aspx.