You can use Advanced Lists to export usernames.
  • Navigate to Lists > Manage Lists.
  • Click the Manage basic and advanced lists block under List Actions. 
  • Click the List Templates tab.
  • Select Template Category: Constituent Information.
  • Locate the template "User Login Information."
  • Click the View/Copy link to the right of the list.
  • This template lists users, with their Login Username.
  • Modify the list if needed, then click Save & Exit.