There are two possible approaches here, both achievable with the APIs, option 1 requires multiple API calls

1. Re-create the certificates directly in Docebo, create and associate the relevant courses, enroll users, and set the enrollment as completed.
2. Upload certificates as External Training.

The choice between these options depends on your requirements and reporting needs.

I assume this is for some external training courses since if you want to grant certificates to users who completed a course that resides on your LMS, you only need to enrol them and make them as completed.

For external training, I found this one that can load the records along with the certificates. You will need o store them some place that is accessible by the API.

https://doceboapi.docebosaas.com/api-browser/#!/learn/ExternalTraining/ExternalTraining_learn_v1_external_training_batch_import