While it is unfortunate that iMovie drops by default from 50fps to 30fps, it also upscales videos to 60fps if the project is started with a 60fps clip! I've ended making myself a one second 60fps dummy clip which I drag first into the project line. After dropping this dummy clip, I verify that the archive export dialog shows 60fps FullHD option, then add the other 50fps clips. At the end I remove the 60fps dummy clip and the project still allows exportation at 60fps.
In this situation iMovie will upscale the 50fps video to 60fps, generating a duplicate frame every five to compensate. Still, this is much better than dropping from 50 to 30 frames.
The most important thing to remember is that the option to export at 60fps is only available if the first clip dragged into the project is recorded at 60fps. If you already have a full project at 30fps dragging the 60fps clip won't help. So remember, drag always the 60fps dummy clip first!