TargetAliases (secondary SMTP addresses) are being created for all selected domains when the Directory Export Wizard is run.
This is the default behavior.
Admin control of Directory Export extraction of Internet aliases for TargetAliases: A new boolean program parameter in Task Parameters and/or Global Defaults lets an admin control whether the Directory Export Wizard will extract users' Internet aliases from the Domino server and insert them into the TargetAliases column of the exported data tables. For example, the default setting:
... enables this feature. The alternative (GenerateAliases=0) disables this feature-tells the Directory Export Wizard to generate only a TargetAddress (the primary SMTP address), but no TargetAliases (secondary SMTP addresses). If the TargetAliases column is populated (either by GenerateAliases=1 or manually), the Data Migration Wizard will load the aliases into Active Directory as it mailbox-enabled Exchange accounts.