QUESTION: I've been shooting in a 16 x 9 format for a short while now. Can I create a project that is 16 x 9 that I am able to add 4 x 3 clips to from some other, older videos I've shot?
 
So, the DVD would be playing in widescreen, but a 4 x 3 clip might cut in for a segment (with black side bars) and then go back to the 16 x 9 format most of the project is comprised of...without squashing or stretching either type of clip in the final project.
 

ANSWER: The best thing to do if you're fine with the black bars (pillar-boxing of a 4:3 into a 16:9 project) is to import that 4:3 video into a project that is set to 4:3.

Then use Clipboard (found under Special button in Edit Menu) to bring that scene/s into the 16:9 project.  Make sure to adjust the settings of the Clipboard to borders and not stretch or panorama

I do have a full tutorial, streaming video with several suggestions and operations to make 4:3 fit better into a 16:9 video here: Mixing Different Aspect Ratios - Weinbar Video Recording