Is there a simple way to import recurring SEPA mandates that have been processed outside CiviCRM previously and should now be managed using CiviSEPA?
Indeed there is not a standard importer for SEPA mandates. I have migrated SEPA mandates a few times and I use the SepaMandate createfull API. That will create the mandate and the recurring contribution.

If you then want to import actual payments you should fill the column contribution_recur_id to ensure they are linked to the mandate.