When you add Slack as a backup to your Pro Backup account, you can only select one workspace in the setup wizard.
Unfortunately, it is not possible to back up multiple workspaces with the same Slack account. You will need to invite a different Slack account for each workspace that you want to back up (as Slack treats each workspace as a separate entity).
Go to Settings > Invite users to invite a user that has access to the workspace that you want to back up.
Important We will never overwrite existing data. Instead we will always create a duplicate. This is true both when you create a copy from your snapshots or when you recover deleted data.