Migrate files into dynamic paths
You can chain a concat process plugin to your destination property so that it contains the result of concatenated source &amp; constant values:

field_paper_working_paper: 
  plugin: file_import 
  source: doc_format_url 
  destination: 
    plugin: concat
    source:
      - constants/destination
      - '/'
      - source_field_value
      - '/'
      - source_field_value
  uid: 1
  rename: TRUE