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As both types of Source Files are 720 x 480, and only differ by PAR (Pixel Aspect Ratio), just Importing the 4:3 into the Project and then dragging it to the appropriate place on the Timeline, should get you the pillar boxing.
Thanks, Bill. Much appreciated.
You are most welcome. That linked article just begins to scratch the surface of the possibilities. I rather like an abstract background, in lieu of the black bars, and often incorporate that with an animation to allow the viewer a visual "bridge"to it.
Good luck, and let us know what you end up doing.