Как преобразовать gridview в datatable в ASP.NET с#?

i have upload one excel file and convert into datatable and bind the datatable in gridview.
i have one button click to add new row in grid view. i want get old data into datatable and adding new row how to do it.
 
i have used below codes to get grid view to data table but this is not working.that codes add only new row old rows are remove or hided.

 sample1
 
DataTable gridTable = (DataTable) dataGrid1.DataSource; 
 
sample 2
 

BindingSource bs = (BindingSource)dgrid.DataSource; // Se convierte el DataSource 
 DataTable tCxC = (DataTable) bs.DataSource;

sample 3

DataTable dt = new DataTable();
 for (int i = 0; i < GridView1.Columns.Count; i++)
 { 
  dt.Columns.Add("column"+i.ToString()); 
} 
foreach (GridViewRow row in GridView1.Rows)
 {
 DataRow dr = dt.NewRow();
 for(int j = 0;j<GridView1.Columns.Count;j++)
 { 
dr["column" + j.ToString()] = row.Cells[j].Text;
 }              dt.Rows.Add(dr);
 }