You an do either/or. Simply overwrite the csf file or add a new directory with the new voice files.

The voice file versions are stored in the M:\V_2\Voices\0506\voice.lst file and also in the M:\V_2\RR\0101\BMWC01S\lng_rel.tbl file

eg: lng_rel looks like this:
09100506Voices/0506/


You cannot load VDO software on the BMW OEM unit - the boot mode coding prevents this from happening.