Have you considered creating a certification containing no courses? Each time a user completes the external training you could upload that in the "Upload certification records" page. Just make sure to configure the certification's "Certification" settings correctly and read all the documentation on the upload page, especially about "Import action", and choose the appropriate option (probably "Certify uncertified users"). Then just use the normal certification reporting.

Or, if you don't need to track the current status of users within Totara (e.g. just having a record and reporting of when users completed the training is sufficient) then maybe you could use either the course or certification upload and enable "Create evidence". Using this method, you could probably do without creating any course or certification in Totara, although this might make reporting across multiple users more difficult (having the records attached to a course or certification allows reporting on that learning item, while recording as "Evidence" might make that harder, if required). Then user the normal Record of Learning reporting.