Aggregating mapping results

Summary

Is there a way to aggregate results of mapping items in a loop other than stage file

Content

In our integrations, we often need to iterate over a collection in a response, mapping each item using data from other responses to a collection in a request.  As an example, in the attached integration I am mapping a quote with line items from one system to another.  I need to get the external system item id  for each item in order to map it to the target line item.  In order to get the external system id for the line item, I need a separate invoke to get the item detail.  Because of this, I can't map the collection as a