It should work. When you import the three components of a live photo (the JPEG still frame, the .MOV video component, and the XPM sidecar file with the metadata) together at the same time into Photos for Mac, and the filenames have not been changed, they should appear stacked as a Live Photo in Photos.
But you have to make sure, that neither the video component nor the still frame are already in the Photos Library. Otherwise the duplicate detection will prevent the reimport of the components as one stacked Live Photo and you will get only a still image and video.
I just tested again and imported these three files together into a small test Photos Library and they appeared as a Live Photo: IMG_2059.heic, IMG_2059.mov, IMG_2059.xmp
Use "File > Import ..." in Photos to import the files together. You can only select the .MOV and the .JPEG files, the XMP sidecar files will be dimmed, but Photos will read and apply them nevertheless, when they are in the same folder as the other two files.

After reimporting the photo is again a Live Photo and I can add the effects like Loop or Long Exposure.
