How do I combine a JPEG and MOV into a Live Photo?
Have you tried adding both files to Photos on a Mac? In a quick test I did, Photos (v2.0) seems to recognize that the files belong together and shows them as a single Live Photo. I found you don't even have to add the files at the same time for this to work. The test I did involved simply taking a Live Photo on my iPhone, copying it to my Mac using Image Capture (which results in a separate JPG and MOV file), and then importing these files into Photos.

A few caveats though:


I’m not sure how Photos recognizes that the files belong together. Presumably this is based on some piece of metadata in the files. If the files you received have been processed in any way, this piece of metadata may have been lost.
In my quick test, I only got this to work with Photos on Mac. I also tried this with Photos on my iPad (iOS 10.3.3), but I wound up with a photo and a movie, rather than a single Live Photo. Once you have the two files on a Mac as a single Live Photo though, you should be able to transfer it as such to an iOS device using AirDrop or some other means like iCloud Photo Sharing.