Use the Sequence Generator transformation to replace missing keys by using NEXTVAL with the IIF and ISNULL functions.
For example, to replace null values in the ORDER_NO column, you create a Sequence Generator transformation with the properties and drag the NEXTVAL port to an Expression transformation. In the Expression transformation, drag the ORDER_NO port into the transformation along with any other required ports. Then create an output port, ALL_ORDERS.
In ALL_ORDERS, you can then enter the following expression to replace null orders: