In a Drupal 6 or Drupal 7 migration to Drupal 8, use the embedded_data migration source plugin to import inline, one-off data. Use this to, say, create new taxonomy terms in a vocabulary. Note the use of the "data_rows" and "ids" keys in the source. "term_name"s are being added to the vocabulary "classes" via the "default_value" process plugin.

upgrade_d6_taxonomy_term_classes.yml

langcode: en
status: true
dependencies: {  }
id: upgrade_d6_taxonomy_term_classes
migration_tags:
  - 'Drupal 6'
migration_group: migrate_drupal_6
label: 'Taxonomy terms for WoW character classes '
source:
  plugin: embedded_data
  # The uid's below are just there for unique keys, they do not carry over to Drupal
  data_rows:
    -
      term_name: 'Death Knight'
      uid: 666
    -
      term_name: 'Demon Hunter'
      uid: 667
    -
      term_name: 'Druid'
      uid: 668
    -
      term_name: 'Hunter'
      uid: 669
    -
      term_name: 'Mage'
      uid: 670
    -
      term_name: 'Monk'
      uid: 671
    -
      term_name: 'Paladin'
      uid: 672
    -
      term_name: 'Priest'
      uid: 673
    -
      term_name: 'Rogue'
      uid: 674
    -
      term_name: 'Shaman'
      uid: 675
    -
      term_name: 'Warlock'
      uid: 676
    -
      term_name: 'Warrior'
      uid: 677
  # "uid" is the key on our data that the migration can use for unique identifiers. Drupal still
  # assigns proper tid's based on available tid's in the system
  ids:
    uid:
      type: integer
process:
  vid:
    plugin: default_value
    default_value: classes
  name: term_name
destination:
  plugin: 'entity:taxonomy_term'
migration_dependencies:
  required:
    - upgrade_d6_taxonomy_vocabulary
    - upgrade_d6_taxonomy_term
    - upgrade_d6_taxonomy_term_instance_size
  optional: {  }