You can import existing students into SMS by registering them in the SMS and binding their SMS account to an existing Moodle account, and then adding the relevant enrollments to their SMS record.
To import students with an existing Moodle account, you first follow the typical registration process. You can register students manually using the staff interface of the SMS, or have students register themselves through the registration page.
When a clerical user sees the new registration and opens the ID verification page, the existing Moodle account will be displayed. To confirm the link to the existing Moodle account, click "Verify ID". If you see a message indicating that a new Moodle account will be created, ensure the email address of the SMS user matches the email address of their Moodle account.
After ID verification, SMS will not yet track any enrollments for that student. You can create new enrollments for that student in SMS which match their existing Moodle enrollments. This will not reset their existing enrollments in Moodle. When SMS synchronizes with Moodle, it will pull their existing progress and grade data into their new SMS enrollment.