How to update the content of an existing collection

I have a situation where I need to combine the content of two separate collections, of different data types, into a single collection that is of a third data type.  I can loop through the first collection building entries in the target collection.  After that I loop through the second collection using the Item In Collection component to find a match on the key field.  At this point how can I copy data from the second collection to the appropriate row in the target collection?