Add missing proxyaddresses attribute to on-premise AD accounts with script

We recently migrated to hosted O365 (Wave 15) with on-site ADFS / dirsync.  All Exchange mailboxes are hosted; no on-site Exchange servers.  Approximately 7500 users.

For each user's proxyaddresses attribute, we need to add the SMTP:username@xxxxx.org, where it doesn't already exist, without modifying any other alias entries already present for the user.

Does anyone have a powershell script that can do this?  I am a newbie powershell user. 

Thanks!  April