4:3 - 16:9
If you drop a 4:3 clip into a FCP 16:9 sequence, it will be pillarboxed
automatically.
If you want to fill the 16:9 frame, use motion to zoom in and/or stretch
the clip. The content and framing of you original 4:3 clip will dictate
how much zooming and stretching you can use without distorting the image.
Another popular choice is some decorative wings. Replace the black
pillars with a neutral color that complements the 4:3 image, or stretch
the 4:3 to 16:9, add some blur, then use the edges as wings. This gives the 4:3 image a soft focus edge filling out to the 16:9 frame.