The first thought that comes to mind is to use Keynote (if you have it). It's an Apple product (similar to Powerpoint) available on the App Store ($20.99 on the Australian Store).
In Keynote, choose a suitable themed blank slide with dimensions of 1920 x 1080 (1280 x 720 also works well). Then drag the 720 x 540 movie onto the slide and scale and position it appropriately (guideline allow you to centre it perfectly). You can then export the project as a movie file (.mov) which can then be imported to iMovie.
If you need further instructions, please post back. There may be other ways of doing it of course, such as using Picture in Picture on a black or coloured background (or video clip) in iMovie - that is perhaps the simplest way, in hindsight.
John