I agree with sbstevesiii.
You can try the following steps
On Photoshop, create a new image with the dimensions of 1600px by 900px (this is the 16:9 aspect ratio) - transparent background, resolution of 72.
Next, roughly calculate what is 1/3 of the image (@533px) and create a selection box of that size (533 by 900) - you can do a set size so with one click you can have that box. Then select your paintbuck - pick the color that you want (white, offwhite, red) and paint that box.
Repeat the step on the other side of the whole image. thus leaving the center transparent
Save as a PNG on your desktop
In iMovie, just drag that image on top of your movie and then select: Picture in Picture
Select the Crop button and click on "Fit"
The move it around to cover your movie - the center should remain transparent, and you can see how it fits your movie.
You can become more creative and adjust your PNG image to cover more by increasing the size of the boxes. You can also get fancy and include your logo and website on that PNG and any other message that can go with your video (creating several PNGs)
Oh I forgot, in the iMovie reel, just select how far you want your PNG to cover. Like I said before, you can create other PNG's based on your template and have them display different things.
Hope this helps. It is not Final Cut, but a workaround if you like to play with Photoshop. I'm actually creating a video in a similar scenario and I was wondering how to handle the black bars. It will be up and running in the next couple of days
Good luck!
Christian