Try this one weird trick with the Migrate API | Horizontal Digital Blog

One of the key concepts of the Drupal Migrate API is the so-called process pipeline, in which we pass a value that is transformed by a series of process plugins. From time to time we find ourselves in the middle of a process pipeline wishing we could easily reference the current value in the process pipeline. I even created an issue on Drupal.org asking for this feature. As it turns out, the feature already exists! That is, as long as you know this one weird trick...