You can merge collections in LUNA v7.1.5+ using the data export feature. The LUNA Library allows you to move media items from collection to collection and export your data as a .csv file from one of your collections so that you can import the data into another collection. It will require a little work on your part to make sure the data you are re-importing adheres to the data schema of the collection you're importing the data to. If the schemas are the same or similar, this will make it easier to do.
Start by exporting the data from the collection you're moving.
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