Generally speaking, you can convert any file archive to another when you extract the original one and compress it again to your chosen format. In case of multi-volume archives, you will need of course all the pieces.
Convert Split Multi-volume ARJ compressed file archive (part 15) to Compressed file from a multi-volume archive (part 8).
We found 2 software records in the database related to .a13 to .008 file format conversion.
Microsoft Windows software - convert a13 to 008 on Windows
Popular open source file archiver with a high compression ratio
Convert from a13 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 a13 to 008. 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