This article will explain how to create a domain wide Send-as Alias on your domain. This can be useful for when you are moving users to a subdomain.
Make a copy of this pre-formatted spreadsheet to generate new email addresses:
Copy and paste the email addresses made by this sheet into the nickname column of the User Directory spreadsheet you already exported from BetterCloud, in column AL:
Change the second email field to that of your sub-domain, and check 'Make this alias the default Send-As address':