default/something.edc:

group {
  name, "subwidget";
  parts {
    ...
  }
}
group {
  name, "mytheme";
  parts {
    ...
    part {
      type, EXTERNAL;
      source, "themed-group";
      description {
        params {
          string, "group" "subwidget";
        }
      }
    }
  }
}