Hi,
Click on the app store icon and type in "Video editing" in the search box. You will see several video editing software programs offered. I am not familiar with any of them myself, so unfortunately have no recommendations. However, there would be no certainty that your clips would work in another video editing app. It might be worth the effort to experiment a little more to see if you can get your clips to import into iMovie without having to re-encode them.
What iMovie version are you using, and what operating system version? What steps are you using to import the clips into iMovie? Describe what happens when you try?
Instead of trying to import your clips directly into iMovie, try importing them to your Mac desktop with the Image Capture app located in the applications folder on your Mac. From your desktop you can then drag them into iMovie. Or, you can import them into the Photos app and from there access them in iMovie through the Photos item in the iMovie media sidebar.
-- Rich