Please refer this example
So if you want to get the parent GridView Rows cell value then pleaes add this line in the Footer Add Button Click event.
string ContactName = ((row.NamingContainer as GridView).NamingContainer as GridViewRow).Cells[1].Text;
You can get any cell value just change the index of cell.
This below line will give you DataKey value of Parent Row.
string key = this.gvCustomers.DataKeys[((row.NamingContainer as GridView).NamingContainer as GridViewRow).RowIndex].Value.ToString();
C# code of Footer Button
protected void AddRows(object sender, EventArgs e)
{
GridViewRow row = (sender as LinkButton).NamingContainer as GridViewRow;
GridViewRow parentRow = row.NamingContainer as GridViewRow;
string ContactName = ((row.NamingContainer as GridView).NamingContainer as GridViewRow).Cells[1].Text;
//This will give you DataKey of Parent Row
string key = this.gvCustomers.DataKeys[((row.NamingContainer as GridView).NamingContainer as GridViewRow).RowIndex].Value.ToString();
string orderId = (row.FindControl("txtOrderId") as TextBox).Text.Trim().Replace(",", "");
string orderDate = (row.FindControl("txtOrderDate") as TextBox).Text.Trim().Replace(",", "");
//Write code here to save these two variable data in SQL table
Response.Redirect(Request.Url.AbsoluteUri);
}