tar cf - <path or file> | 7zr a -si <output>.tar.7z
This means: tar "c" creates an archive, to "f" file (or device), in this case stdout ("-", the minus), all piped to 7-zip, here 7zr, reads from stdin ("-si") and outputs a 7zip file - all connected by the pipe.
An interesting variant might be to pipe tar output to lz4. lz4 is a little command line tool implementing the LZ4 compression scheme. It's very fast, similar to Google's Snappy/Zippy, and provides a light compression without slowing down write throughput that much:
tar cf - <path or file> | lz4 - <output>.tar.lz4
More file archiving/compression posts: