I use MPEG Streamclip (free and cross-platform) to save large files into smaller chunks. You can set "in" and "out" points and save the file as smaller pieces without re-encoding. It simply trims the front and back when you use the "save as" option. This makes the process very quick and non-destructive. Although with that large a file it will still take a while to save it.
From there you can more easily manipulate the smaller chunks. You may still need to re-encode into a format that iMovie supports. MPEG Streamclip can also convert to a variety of formats.