If you want to convert compressed archive from one to another, let's say from 7-zip to Unix TAR, the best course of action is to decompress the first archive and then compress it again to different format. Good file archives support many different format, which should make the job very easy.
Convert 7-ZIP compressed 7z archive to Unix standard Archive format, Tape Archive.
We have found 5 software records in our database eligible for .7z to .tar file format conversion.
Microsoft Windows software - convert 7z to tar on Windows
Popular open source file archiver with a high compression ratio
Convert from 7z file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned 7z to tar. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
One of the many available file compression utilities
Apple macOS / Mac OS X software - convert 7z to tar on OS X
A compression utility for OS X
A compression utility for Mac
Mac version of the popular utility