This isn't the cleanest/quickest/easiest/most elegant solution, but it is a brute force one that I created to get the job done in a similar scenario:
DataTable dt = (DataTable)Session["dtAllOrders"];
DataTable dtSpecificOrders = new DataTable();
// Create new DataColumns for dtSpecificOrders that are the same as in "dt"
DataColumn dcID = new DataColumn("ID", typeof(int));
DataColumn dcName = new DataColumn("Name", typeof(string));
dtSpecificOrders.Columns.Add(dtID);
dtSpecificOrders.Columns.Add(dcName);
DataRow[] orderRows = dt.Select("CustomerID = 2");
foreach (DataRow dr in orderRows)
{
DataRow myRow = dtSpecificOrders.NewRow(); // <-- create a brand-new row
myRow[dcID] = int.Parse(dr["ID"]);
myRow[dcName] = dr["Name"].ToString();
dtSpecificOrders.Rows.Add(myRow); // <-- this will add the new row
}
The names in the DataColumns must match those in your original table for it to work. I just used "ID" and "Name" as examples.