Success! 
With @pfaffman’s guidance, deleting import_id from both Discourses before export/import worked:
rails c
UserCustomField.where(name: 'import_id').delete_all
GroupCustomField.where(name: 'import_id').delete_all
CategoryCustomField.where(name: 'import_id').delete_all
PostCustomField.where(name: 'import_id').delete_all
This did not work:
discourse import_category /category-export-2017-10-25-225306.json
but this did:
bundle exec discourse import_category /category-export-2017-10-25-225306.json
Having to delete import_id from the Discourse that was converted with the phpBB importer prevents me from doing a leisurely initial import first and an incremental import later, but I can live with that.