Add column at specific position in a datatable

I have a datatable which is used to convert the data to a CSV.
The last column is used to get dynamic data which is get a list of all orders a customer has placed (this means cust 1 could place 2 order items and cust 2 could have 5 order items). The data would display as
Cust Name, OrderID, OrderItem1, OrderItem2, OrderItem3, OrderItem4, OrderItem5 Cust1          - 1          - cd               - DVD Cust2          -2           - Chocolates - Laptop     - DVD          - Biscuits        - Pepsi
So as you can see its one line for each order displaying each order item and a column is created adding the order item count if it doesnt already exist.
The code to get the order items is in a foreach loop which creates the column dynamically.
So i need an ...