You can do this and I've done it in the past. The biggest problem you have is if the approvers only have access to self service. Because a user can only see and update their tickets it means you would need to update the customer information and effectively transfer the request to be under the approvers name. Then they could edit the request, save it and it would continue with the workflow. (and probably changing the request back to the original customer's name).
If the approvers have a service desk analyst license then they would be able to do this via the service request module as they would a normal ticket, just updating the request once they receive an approval notification.
Hope that helps.