Hello,
You can use the KeyDownHandler of ClientSideEvents and use the JS function to add new row and get focus on cell with methods scrollToView and activate. Plase taka e look at the code below:

<ClientSideEvents KeyDownHandler="Action" />


  <script type="text/javascript">
        function Action(gridID, cellID, key) {
            if (key == 9) { // 9==TAB key number
                var grid = igtbl_getGridById(gridID);
                igtbl_addNew(gridID, 0)
                var getNewCell = "UltraWebGrid1_rc_" + (grid.Rows.length - 1) + "_0";
                var cell = igtbl_getCellById(getNewCell);

                cell.activate(true);
                cell.scrollToView(true);

                //to do some manipulation with the cell

            }
            return true;
        }
    
    </script>

Also you can take a look at the link below:

http://samples.infragistics.com/2008.2/webfeaturebrowser/default.htm
Grid -> Database Updating

Hope this helps.