Add email alias to multiple O365 user accounts in bulk

So simply adapt the script to run against just the users in question, either by using filter or by feeding a CSV file to the script. Here's a simple example:

Import-CSV blabla.csv | %  {Set-Mailbox $_.UserPrincipalName -WindowsEmailAddress $_.NewAddress }

where the blabla.csv file has a column UserPrincipalName to identify the users, and a column NewAddress to specify the new primary SMTP address.