If your application is using the Code First approach you could easily set the DataAnnotation attributes directly on the properties of your entities.
Otherwise in order to use the DataAnnotations with the Telerik OpenAccess ORM we are offering a custom code generation template which is generating buddy classes for your entities. Also this template is automatically generating the Key, Required and StringLength attributes for your properties based on the metadata from your database.
After creating these classes you could easily delete the code generation template, add any other needed attributes for the properties and use these buddy classes in your application.
You could find more information about this template and how to use it in this Code Library example.
I hope this helps. Do not hesitate to contact us back if you need any further assistance.
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.