I the following script to auto create support cases:

var newrecord = nlapiCreateRecord( 'supportcase' );
newrecord.setFieldValue( 'assigned', a );
newrecord.setFieldValue( 'company', b );
newrecord.setFieldValue( 'title', c );
newrecord.setFieldValue( 'incomingmessage', d );
var caseid = nlapiSubmitRecord( newrecord, false, true );
// this script will be used in an automated process, hence ignoring mandatory fields and sourcing

This works terrifically, creating a new Case and assigning 'incomingmessage' to a new message attached to the newly created case.

As a followup I tried to attach a second message to the case with the following script:
var record = nlapiLoadRecord( 'supportcase', caseid );
record.setFieldValue( 'incomingmessage', 'All your base are belong to us.' );