How to add New Column with Value to the Existing DataTable?

Add the column and update all rows in the DataTable, for example:

DataTable tbl = new DataTable();
tbl.Columns.Add(new DataColumn("ID", typeof(Int32)));
tbl.Columns.Add(new DataColumn("Name", typeof(string)));
for (Int32 i = 1; i <= 10; i++) {
    DataRow row = tbl.NewRow();
    row["ID"] = i;
    row["Name"] = i + ". row";
    tbl.Rows.Add(row);
}
DataColumn newCol = new DataColumn("NewColumn", typeof(string));
newCol.AllowDBNull = true;
tbl.Columns.Add(newCol);
foreach (DataRow row in tbl.Rows) {
    row["NewColumn"] = "You DropDownList value";
}
//if you don't want to allow null-values'
newCol.AllowDBNull = false;

Without For loop:

Dim newColumn As New Data.DataColumn("Foo", GetType(System.String))     
newColumn.DefaultValue = "Your DropDownList value" 
table.Columns.Add(newColumn) 

C#:

System.Data.DataColumn newColumn = new System.Data.DataColumn("Foo", typeof(System.String));
newColumn.DefaultValue = "Your DropDownList value";
table.Columns.Add(newColumn);