this.radSpreadsheet1.SpreadsheetElement.InputHandler = new CustomSpreadsheetInputBehavior(this.radSpreadsheet1.SpreadsheetElement);

        public class CustomSpreadsheetInputBehavior : SpreadsheetInputBehavior
        {
            public CustomSpreadsheetInputBehavior(RadSpreadsheetElement spreadsheet) : base(spreadsheet)
            {
            }

            public override void ProcessKeyDown(KeyEventArgs e)
            {
                if (e.Shift)
                {
                    this.Spreadsheet.ActiveWorksheetEditor.Commands.EnterActiveCellEditMode.Execute(e.KeyCode);
                }
                else
                {
                    base.ProcessKeyDown(e);
                }
            }
        }