MPEG Streamclip can do this. It’s a freeware tool that you will probably need to use anyway to transcode the files from H.264 / MPEG-4 AVC to the Apple Intermediate Codec. (Unless you import directly from the connected camera in iMovie, where iMovie is going to transcode automatically.) If you don’t want any transcoding, you better have a not-too-old or low-spec Mac because the highly compressed H.264-encoded video stream is extremely processor-intensive in processing.