Add fields to a search index

$index = search_api_index_load('person-index', TRUE);

$fields = array(
  'options' => $index->options,
);

$fields['options']['fields']['example_integer_field_name'] = array(
  'type' => 'integer',
);
$fields['options']['fields']['example_text_field_name'] = array(
  'type' => 'text',
);

search_api_index_edit($index->id, $fields);

 The array keys (like example_integer_field_name) are the property paths or machine names for the fields – e.g., author:name for the Author name. The valid types are returned by search_api_get_data_type_info(). Note, though, that for multi-valued fields you’ll have to use list<TYPE> instead (or even list<list<TYPE>> in case of two multi-valued properties in the property path – like field_tags:parent.)