MKV is a container file format that can hold an unlimited number of video, audio, picture, or subtitle tracks in one file. It is a universal format for storing common multimedia content, like movies or TV shows.
After you have downloaded some MKV videos from the Internet, you may somethings need to edit the MKV files, then, iMovie, as a free and popular video editing program will inevitably become your ideal choice.
However, as iMovie natively can't edit MKV files, it is impossible for you transfer MKV videos to iMovie directly. Then, you will need to find an alternative way to solve the problem. And as far as I know, the most easy way is to find a Mac Video Converter to convert MKV files to iMovie compatible MOV or MP4 files.
Then, you are advised to use MKV to iMovie Converter to help you convert your MKV files to iMovie (08'/09'/11') workable MOV or MP4 files. Besides, it is also able to convert other videos to iMovie, such as AVI to iMovie, MPG to iMovie, WMV to iMovie, AVCHD to iMovie, MXF to iMovie etc.
Step by step to convert MKV to iMovie
Step 1: Import MKV files
You can load MKV files by clicking "File" > "Add Video Files/Add Audio Files/Load DVD" on the menu bar or dragging the files directly.
Step 2: Select iMovie as output format
Click "File" > "Encode Settings", or click the "Settings" button to name the output file, define iMovie as its format and codec, rates, etc
Step 3: Convert MKV files on Mac
Click "Convert" to start MKV to iMovie Conversion. After that you can free import MKV videos to iMovie for free editing.