Well, if you have all pieces of the multi-volum arcive, you can simply decompress it, and repack to your preferred format.
Convert Split Multi-volume ARJ compressed file archive (part 11) to Compressed file from a multi-volume archive (part 8).
We found 2 software records in the database related to .a09 to .008 file format conversion.
Microsoft Windows software - convert a09 to 008 on Windows
Popular open source file archiver with a high compression ratio
Convert from a09 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 a09 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