Hi Eyal Goren,
This option is not available in the designer. Actually at the moment we are working on the area of identity handling and you might not be happy to read this but our plans are to remove the requirement for ID class generation. Both single or multiple-field identities will be handled internally. This way we will avoid a lot of limitations that were caused by the usage of ID classes. Of course, the already generated classes will continue to work for backward compatibility.
Having this in mind, if you would like to migrate to a domain model it would be better if you can avoid the dependency to the ID classes. If you have any doubts how to achieve that, please give us some details about your scenario and how you are using the ID classes, we will do our best to provide you with a solution.
the Telerik team