Add additional username to person document using CMN DirSync

Hey there, I know that it is possible for MNE to add the routing mail to the username field during migration. (Trevor answered this here)

However our use case is a bit different. We are still in the migration process, but new users are already only created in AD / Exchange Online. This requires CMN DirSync to write the contacts back into the Domino directory. How do I add the routing address as an additional usernames within the DirSync process to the respective user?