Dear Jove,
I suspect you are asking something along the lines of how you make 16:9 film fit a 4:3 aspect ratio without distorting the film, so your get the tell tale sign the the 'black' bars top and bottom?
This is not as easy as you think. It all depends on the software that you using. Unfortunately, you only gave iMovie, which I don't use unfortunately. I use Final Cut Pro or Premiere, which is these programs is fairly simple.
The general idea is to set the stage of the final dimension of your movie, and then import the film and superimpose it on top. In FCP and Premiere, you have these controls to stretch and shrink the film into place, like Free Transform in PhotoShop. But in Final Cut Pro, there is a command that does this for you. In the end, you have two video tracks, one that has the 'black', which is a continous track of just black, and the other track you have your film, stretched or shrunk so the edges of the film reach the edges of the final dimension of the movie.
(hope that made some sense)
I have no idea how this is done in iMovie, sorry. You might want to invest in Final Cut Express?
Sorry I wasn't more help in respect to iMovie.
There might be a third party solution, like some freeware or plugin that does what you ask, but I am unaware of any.
Good luck...
PS. There is a screenshot to show you how it's done in Final Cut Pro 3.0.4. Hope it helps...