First up, get rid of these lines because they're pointless:
Code:
            da.InsertCommand = cmdbuild.GetInsertCommand();
            da.UpdateCommand = cmdbuild.GetUpdateCommand();
            da.DeleteCommand = cmdbuild.GetDeleteCommand();
As for the question, I have no idea why you're Filling a new DataTable and importing its rows into an existing DataTable and then saving the existing DataTable. That's not going to make any changes to the existing DataTable that can then be saved. Is that what you're expecting to happen? I think you need to explain to us how this is supposed to work.