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.
Now, open the .csv file of the data you exported and check if the field headers match the field headers of the collection you are importing to. If they do not match you'll need to make adjustments to the .csv file so that it will upload successfully. Visit LUNA Uploader to learn how to prepare a .csv file for upload.
Next, copy all images from your first collection into the collection you want to merge with by selecting "Add to collection" in the More menu.
Once all media items have been moved, the last step is to upload the data you exported from the first collection. To link the images to the data, select the appropriate linking field and the option for "Any media items and records in collection (slow)".
Depending on how many images are in the collection you're moving, you may choose to work in batches.