As far as I know you'd have to use javascript to do this.

Java script:

function OnUwgKeyDown(gn,cellId,KeyStroke)
{
    // Tab Key
   if (KeyStroke == 9)
   {
        SelectNextCell(gn);
        return true;
   }
}

function SelectNextCell(gridClientId)
{
    var grid = igtbl_getGridById(gridClientId);
    var cell = grid.getActiveCell();
   
    if(cell != undefined && cell != null)
    {
          var nextCell = cell.getNextTabCell();
         
          while(nextCell != undefined && nextCell.isEditable() == false)
          {
             nextCell = nextCell.getNextTabCell();
          }
                   
          if(nextCell != undefined)
          {
              cell.setSelected(false);
              nextCell.activate();
              nextCell.setSelected(true);
          }
    }
}

Grid: