Is it possible to make a cell editable while in add new row mode and make readonly when its in Edit row cell
You don't need to allow cell editing for the add new row to work, so if the only row that will allow entry is the add new row, just don't turn on editing for the grid.
Otherwise you could simply put a check in the CellEnteringEditMode and check the e.Cell.Row.RowType to see if it's a RowType.DataRow and if it is set the e.Cancel event to true.