protected void addExtension(LocalRegion dataRegion) {
    LuceneIndexCreation creation = new LuceneIndexCreation();
    creation.setName(getName());
    creation.addFieldNames(getFieldNames());
    creation.setRegion(dataRegion);
    creation.setFieldAnalyzers(getFieldAnalyzers());
    creation.setLuceneSerializer(getLuceneSerializer());
    dataRegion.getExtensionPoint().addExtension(creation);
  }
Register an extension with the region so that xml will be generated for this index.
geode-lucene/src/main/java/org/apache/geode/cache/lucene/internal/LuceneIndexImpl.java