Extension fields must first be added in c4c before the migration template.
Go to the ui screen of the relevant object, say accounts.
Open any account.
Enter adapration mode through the adapt button on the top of the screen.
Navigate to to the extension fields section.
Create there. Publish.
Go to the migration tool for accounts.
you will see a pop up that the template will be automatically adapted with new fields.
Post that download the new tenplate.
Thank you.