You should be able to accomplish this by manually adding colspan attribute during the data bound event.

See below example for where I use a boolean data field named isSpecial to determine if that row needs to be altered:

function onDataBound(e) {
  var grid = $("#myGrid").data("kendoGrid");

  // Iterate through all the rows
  var rows = grid.items();
  $(rows).each(function (e) {
     var row = this;
     var data = grid.dataItem(row);

     // Check if this row needs to be altered
     if (data.isSpecial) {
        // Set the column span
        $(row).find("td:first-child").attr("colspan", "5");
     }
  });
}