MultiFileImport.ImportSettings settings = mfi.getImportSettings(); // gets a copy of “ImportSettings”
List<File> currentImportDataFiles = settings.getImportDataFiles(); // current existing files in Multi File Import
List<File> newImportDataFiles = … // add new Files
for (File file : newImportDataFiles) {
 if (currentImportDataFiles != null && currentImportDataFiles.contains(file) {
   continue; // file exists
 }
  settings.addImportDataFile(file);
}
mfi.setImportSettings(settings);