You cannot convert individual parts of a split ZipSplitter archives, such as .s05 to .s01. The only conversion possible would be to gather and merge all the pieces and get either a single archive or the original file(s) back. Not a conversion in proper sense, but doable, you need all the pieces though,
Convert Split Multi-volume ZipSplitter compressed file archive (part 6) to Split Multi-volume ZipSplitter compressed file archive (part 2).
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert s05 to s01 on Windows
A program for splitting small files
Convert from s05 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.