Geed thanks for your reply.
I am afraid it's a little more complicated with iMovie.
iMovie does recognize and exports 2.7K video, just not when it's in a project on a timeline. If you select an imported 2.7K clip, it will export as 2.7K (it says 4K but shows the correct 2.7K resolution).
It goes wrong when you add the 2.7K clip to a project/timeline.
You can even use a workaround by placing a 4K clip (just a second is enough) as the first clip in a project, iMovie recognizes 4K and the project becomes a 4K project, if you then add all your 2.7K clips, edit the project and remove the 4K clip, it will still show as a 4K project and you will be able to export your 2.7K footage as a 4K movie.