iMovie is part of Apple's iLife suite and lets you view and work with, edit and share your movies. The latest version of iMovie has made a marked improvement in working with large projects, reliability and performance. Your movies, clips and photos are automatically organized in the built-in library so they're just a click away. iMovie has an innovative interface that makes it quick and easy to look through the library and to create new movies. With a few simple steps, you can import movies to your site, put them up on YouTube and even create clips for your phone, iPhone, iPod or Apple TV.
iMovie supports both standard and high definition videos in a number of video formats including MPEG-2, MPEG-4, Quick Time Movie, AVCHD, DV and HDV(High Definition Video), Mac Video Converter is the best MTS to iMovie Converter that can convert mts to imovie mp4 format, then you can import mts to imovie.
Step 2: Select output format
Choose a format like "MPEG-4 Movie (*mp4)" which is compatible with iMovie from the profile drop-down menu. 
Step 3: Convert MTS to iMovie MP4
Click "Convert Checked Item(s)" button on the toolbar to start converting MTS files to iMovie MP4 format.
Step 4: Importing convertion mp4 files to imovie
Launch iMovie, go to "file > import movies". Select the converted MP4 video then click "import".