You can use PeaZip software that is able to extract data from WinRAR compressed archives (also supports WinRAR split archives) and subsequently re-pack these data to 7-zip compression file format.
Convert WinRAR compressed archive file to 7-ZIP compressed 7z archive file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert r0 to 7z on Windows
A free open source archive utility
Convert from r0 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.