Start Routine in Update Rules to populate additional fields from ODS

I have a flat file load which only contains values for a few characteristics on an order line item. I need to populate about 75 other fields for that order line item.

These 75 other fields are already assigned to that order line item - they exist in the ODS that this flat file load is going into. If the order item is >=2, I need to perform the population of the fields.

How do I write a start routine to assign all of these fields to the order line item?

The purpose of this is basically to create a new complete record.

Thanks,

Darryl