I found the solution! At least, for me the following process is working:
- I create my animation in Apple Motion (I use version 5.5) on the Mac (Catalina 10.15.7). I have no background layer and in the project's properties the "Background" setting is set to "Transparent".
- In Motion I use the Share button and the "Export movie (default)..." entry to initiate a rendering of the project. On the Settings tab, I set the "Format" to "Video and Audio" (or "Video only"), the "Video Codec" to "Apple ProRes 4444" and the "Color Channels" to "Color + Alpha". Then press "Next..." and chose a location to save the .mov file.
- I open the created .mov file in the Quick Time Player app on the Mac and use the File > Export As ... > 1080p (for example) command. In the upcoming dialog be sure to check the two checkboxes "Use HEVC" and "Preserve Transparency", click "Save".
- Send this newly created .mov file via AirDrop to your iPad and import it into your LumaFusion project and – Voilà! – enjoy your beautiful & professional animation as an overlay with transparency in your LumaFusion project!
Hope this helps someone and works for you, too.