In order to transfer user's completed certifcation the following conditions should be satisfied.
1. Certification should have a valid External ID
2. User record should have a valid External ID (Contact ID)
3. User's user groups should have valid external ID.
4. User should complete the certification.
When all these conditions met, certificate will be automatically transfer to SalesForce. In addition LMS Admin could click the "Send user Certification to SFDC" from User's Manage Course Registration page.