Convert tar to 7z directly?

I have a couple tarballs that are many gigabytes large and contain several thousand files and am wondering if it is possible to convert these to more versatile 7z archives, because the current tarballs make pulling individual files out of them very tedious. If at all possible I would like to do this in-place, i.e. without having to fully extract the tarballs. I thought I could do it with the following command, but the resulting archive just contains a single file with all the data.

tar -xf 100GB.tar -O | 7za a -si -ms=16f32m 100GB.7z

Is this somehow possible?