Unfortunately at the moment this is the only way to add Authorization Attributes - manually in the generated domain service cs file. We would only recommend you after you generate the service class, to remove the template which generates the service cs file to protect from possible regeneration that could wipe out your changes.
As the definition of attributes in the RIA services in the RIA wizard is a valid feature request, you can also post a request to our Ideas and Feedback portal so there would be an easy way to add Authorization Attributes as part of the code generation.
Do not hesitate to get back to us if you have any other questions.
the Telerik team