You can use any video converter listed in our conversion list as well as multi-platform FFmpeg transcoding tool. To transform MPEG-4 video file format (.m4v) to 3GPP2 multimedia file (.3g2) with FFmpeg → open command line and type:
ffmpeg -i /path/to/your/file.m4v -vcodec copy -acodec copy /path/to/your/newfile.3g2