How to use 7z to compress via pipe?

This is covered in the man page of 7z:

-si    Read data from StdIn (eg: tar cf - directory | 7z a -si directory.tar.7z)

Is there a reason you want to use 7z specifically, or do you just want better compression than gzip?

The xz utility uses the same compression algorithm as 7z (LZMA), and allows piped compression the same as gzip.

tar cvf ... | xz -9 | openssl ...