Conversion of AR files to 7zip format (.7z) does not make much sense. However, some of the known .ar files are archives (mainly ZIP) and thus can be extracted using 7zip and repacked again should you want it. But a conversion seems rather pointless.
Convert Midtown Madness city add-on file to 7-ZIP compressed 7z archive.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert ar to 7z on Windows
Popular open source file archiver with a high compression ratio
Convert from ar 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.