Hi @IsabellaFaro,
So I was assuming based on your initial post that you had an integration populated for each user, but sounds like this may not be the case after all. In order to use the sis method, you'd need to have either an existing sis_id or integration_id. Without either of those pieces in place right now, it gets a bit trickier to add an sis_id.
What I would suggest in this case is to run a provisioning report for users from your root canvas account. You can then open that file and populate the user_id column with the desired sis_ids (there is definitely some confusion between a canvas_id and an sis_id in different areas of Canvas), save the file, and upload it to Canvas. This should get the sis_id populated for the existing users. I tested this in my instance before posting just to verify it still worked.
Hope this helps!
-Chris