Vince,
If your Unity usernames are in the same format as your email addresses, you should be able to export all of the users via BAT and then in excel use this formula in the SmtpAddress_TextPageer1 field:
=A1&"@companyname.com"
where A1 is the username field. This will combine the value in A1 with @companyname.com.
Save the excel file as a CSV and run a user update in BAT.
should take you less than 5 minutes.
Jeff