Yeah, I’ve used boris fx to do it.
Change the face of the media track to 16:9 aspect, and then scale the thing to 70% to fit it on 4:3.
Tricky thing is getting the footage in.
I usually export by ref,
then create a composition clip in m100, double click it, import the by ref file, change the media face to 16:9, and change the X/Y Scale under position, to 70% or %75, depending how it looks, then quit and save settings. Then stretch the composition clip to the full length of the imported clip…or just type in the length and hit enter, and then hit render. (If you give the Comp Clip a name before rendering, you can then search for the name of that file in your media folder, and import that back into your M100 bin as a legitimate green clip, that won’t accidently get unrendered later.)
Then you can export your newly rendered clip as your letter box file, or just play it as a letter box in the timeline/whatever.
Bear in mind, even if your original anamorphic 16:9 was exporting as a squashed 4:3, it would play back as true 16:9 on a 16:9 monitor.