For a Drupal 7 to Drupal 8 migration, we're taking what was a Quotation content type and migrating it into an Article content type, joining a ton of nodes that are staying as Articles. To distinguish, I want to add a taxonomy term "Quotation" to the field_categories (which exists on Articles D7 and D8 but not on Quotation).
So i'm "simply" trying to add a value not from a source, but hard-coded. My source plugin is d7_node (namespace Drupal\node\Plugin\migrate\source\d7).
I'm flailing a bit; tried entity_generate but couldn't figure out how to tell it to use a constant and/or default value rather than looking for something from source.
Even this didn't work, when i tried to test the simplest thing i could think of when already knowing the term ID of an existing term:
Can one use the default_value plugin or something like it for the 'source:'?
I'm about to stick my non-source value into what comes from the source with a prepare row hook, but it seems there ought to be an easier way.
Note that in this case there is nothing from field_categories in the source to worry about, but i'm also interested in the case of adding a new, hard-coded-into-the-migration term into a field that will have content, in my case field_tags.