Hi,
As a precaution to prevent accidently sending lots of emails after an import it's not possible to subscribe the users to a follo up at the time of importing them.
To achieve this you can run a mass action. Choos we"Subscribe to a follow up campaign" as Action and choose your import value (Acymailing field->Source->the value of your import) as Action Targets.
Mass actions are part of the Automations feature that comes with the Enterprise version.