Isnt a DataSet a collection of DataTables ?
I know you can do this :-
DataTable dt21 = new DataTable("productname");
DataColumn theCol = new DataColumn("Groups", typeof(string));
dt21.Columns.Add(theCol);
da2.Fill(dt21, "productname");
and then you'd likely do this :-
DataSet set = new DataSet("productmaster");
set.Tables.Add(dt21);
for example
....
'g'