Aggregator with custom RSS field
Aggregator offer the possibility to add custom fetch/parser/process to update/override/remove actions on the global process. 

For example I managed to make aggregator deal with enclose url image. 
Their are several ways to do it with Drupal 8. I used a very simple one : Annotation.

/**
 * Defines a default parser implementation.
 *
 * Parses RSS, Atom and RDF feeds.
 *
 * @AggregatorParser(
 *   id = "xxxxx_aggregator",
 *   title = @Translation("XXXX parser"),
 *   description = @Translation("Custom parser for RSS enclosure tag.")
 * )
 */
class CustomParser extends DefaultParser {

  /**
   * Override default parser to add the enclosure attribute to feed item
   * {@inheritdoc}
   */
  public function parse(FeedInterface $feed) {

    parent::parse($feed);
    // your code...
  }

}


In this given example, using @AggregatorParser, I allow Aggregator to detect and use another PARSER, my custom, which is extending the aggregator's default.

Do not forget to add it in the correct needed path. Here its : namespace Drupal\xxxxx_aggregator\Plugin\aggregator\parser;

You will be able to see and enable it on the aggregator feed admin page. 

For deeper informations you can analyse the aggregation process throught the module's files, this contrib module use plugin management and hooks api. AggregatorPluginManager.php