A MOD file is similiar to a Mpg file. I've never worked with MOD filed before, so not experienced with them.
Try making a duplicate of your MOD file and see if the VLC app on your Mac will convert it to Mp4/AAC. Look in VLC's Custom settings to choose the audio and video format. If you can convert the MOD file it should play in iMovie.
You can also try converting the MOD clips to H.264, Mp4/AAC with the free download, Handbrake. I'm not sure whether Handbrake can convert MOD clips, but it might be worth a try.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich