You can split large archives (like .rar) and produce a multi-volume archive, which in some versions produce 001, 002, 003 etc file extension sequences. It's not much of conversion, but more of a standard feature of most archiving tools.
Convert WinRAR RAR compressed archive to Compressed file from a multi-volume archive (part 1).
We found 7 software records in the database related to .rar to .001 file format conversion.
Microsoft Windows software - convert rar to 001 on Windows
Popular open source file archiver with a high compression ratio
Convert from rar 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 rar to 001. 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 oldest and most popular shareware file archiver and data compression utility
A free open source archive utility
Apple macOS / Mac OS X software - convert rar to 001 on OS X
A backup and archive utility
Mac version of the popular utility
Linux/Unix software - convert rar to 001 on Linux
A Linux application used as frontend for popular compression tools
A file archive manager