Drag your 4:3 NTSC movie into Compressor.
Use the Preset (in Advanced Format Conversions) for DVCPROHD 720p60.
In the Inspector, go to the Geometry tab and go to the bottom where it says “Output Image Inset (Padding)” select 4:3 – 1.33:1.
Thats it.
To preview the Pillarbox (to make sure you did this right…go to the upper right corner of the Preview window in Compressor and press the button in the upper right corner. Be sure to slide the split-screen slider above the preview window all the way to the left to see a preview of the entire encoded frame. You may have to toggle back and forth, as sometimes Compressor’s preview does not update right away. You should see your image pillarboxed. At least it works here.