It will be a lot easier to do it either from front end if you use datacontrol or from back end in T-SQL query with a Row_number function.

If both options are not your choice, you can work with your datatable object directly through datatable.Load method.

Here is the code snippet to add autoincremental number column to an existing datatable:

 private static DataTable AddAutoIncrementColumn() 
        { 

            DataColumn myDataColumn = new DataColumn(); 
            myDataColumn.AllowDBNull = false; 
            myDataColumn.AutoIncrement = true; 
            myDataColumn.AutoIncrementSeed = 1; 
            myDataColumn.AutoIncrementStep = 1; 
            myDataColumn.ColumnName = "autoID"; 
            myDataColumn.DataType = System.Type.GetType("System.Int32"); 
            myDataColumn.Unique = true; 

            //Create a new datatable 
            DataTable mydt = new DataTable(); 

            //Add this AutoIncrement Column to a new datatable 
            mydt.Columns.Add(myDataColumn);            

            return mydt; 

        } 

protected void Page_Load(object sender, EventArgs e) 
        { 

            DataTable myDataTable = new DataTable(); 
            myDataTable = AddAutoIncrementColumn(); 

            //Read your existing datatable into a datatableReader 
            DataTableReader reader = new DataTableReader(GetTable()); 
            myDataTable.Load(reader); 

            GridView1.DataSource = myDataTable; 
            GridView1.DataBind(); 

        }

http://msdn.microsoft.com/en-us/library/7x8ccbsb.aspx

Another way is to add a column and loop through the datatable to add the counter:
(From this thread: http://forums.asp.net/t/1712346.aspx/1?add+serial+number+in+existing+datatable )