In general you could use domain method in the code-behind of your page. In order to do that you could handle some of the various OpenAccessLinqDataSource events that are used to customize the behavior of the control.
For your convenience I created a sample application demonstrating how to handle the Selecting event to populate the data source from domain method, please find it attached. Bear in mind that the example is using the AdventureWorks database and its upsGetEmployeeManagers stored procedure.
Let me know whether this helps or if I can assist you further.
Kind regards,
Dimitar Tachev
the Telerik team
Telerik OpenAccess ORM Meets ASP.NET Web API. Read more.