[itsme]

No, I cannot control that.
The song title format allows specifying the separator between fields, but not the separator between list entries.

e.g. a song with several composers:
The CSV file
Title;AlternativTitel;PDFPage;Pages;SourceTypes;Keys;Years;Composers;Tempos;Signatures;Albums
Achin' Hearted Blues;;331;331-332;LS|2 S.|Tx;Eb;;Clarence Williams|Clarence Johnson|Spencer Williams;;4/4;CJB Fakebook 2023
also works with tabs between the fields and creates three composer entries in the MobileSheets database as expected.

Exporting with this setlist format

%TITLE%;%CUSTOM_GROUPS%;%KEYS%;%PAGE_ORDER%;%FILE_NAMES%;%SOURCE_TYPES%;%FILE_PATHS%;%COMPOSERS%;%GENRES%;%YEARS%;%SIGNATURES%;%TEMPOS%

creates this

CJB Fakebook 2023 Sample

Achin' Hearted Blues;;Eb;331-332;_CJB_Fakebook_2023_C.pdf;2 S., LS, Tx;D:\HerbertsGoogleDrive\_MobileSheets\Fkbk0\CJB_Fakebook_2023\_CJB_Fakebook_2023_C.pdf;Clarence Johnson, Clarence Williams, Spencer Williams;;;4/4;

Simply copying the setlist format into the file should create a valid CSV file for import. But re-importing that creates one composer entry, not three.
The commas between the composers for the export as well as the | for CSV import seem to be hardcoded. Using search&replace or a script to replace comma with | is not possible because composers often are specified as
Williams, Clarence|Johnson, Clarence|Williams, Spencer;
to get them sorted by lastname.