If you want to import the converted file into iMovie, first, the file format should be Quicktime-compatible, like MPEG-4, DV, or .mov.
Please refer to the following steps:
1. Open iMovie. Go to File on top menu > Import Movies.
2. A Finder window will open. Browse to the video file you want to import.
3. Select Copy Files, (also on the Import dialog box), if you wish to copy the files to the new location while keeping the old files where they were. Select Move Files, if you wish to move the files to the new destination and remove them from the previous location.
4. Click Import button.