Hi Deskside,
The short answer is yes. During the staged migration, you can create new on-premise mailboxes during the staged migration process.
In a staged migration, we migrate the mailboxes listed in a CSV file. After the mailboxes in the migration batch migrate to Office 365, we change the on-premise mailboxes to mail-enabled users so that Outlook can connect to the online mailbox after users create a new profile.
Best Regards,
Bruce Zhou