7z An Existing Tar File

I am on ubuntu 20.04 and I have a 71GB.tar file that I would like to 7zip before moving it to long-term storage. I can create a new directory.tar.7z using: #tar cf - directory | 7z a -si directory.tar.7z However, I cannot #7z a -si directory.tar.7z It just freezes.... no errors... How can I 7zip *pre-*existing tar archives?