How do I migrate content to a field added with UI to an existing custom content entity?
In fact, it was easier than I thought. I just had to add the field to the Person migration file:  

id: migration_persons_with_sel
migration_tags: null
migration_group: null
label: 'Person migration from CSV'
source:
  plugin: csv
  path: /Users/efalzon/Sites/JdP8M7b/config/migration_Persons.csv
  delimiter: ;
  enclosure: '"'
  header_row_count: 1
  keys:
    - id
process:
  id: id
  lastname: lastname
  firstname: firstname
  cellphone: cellphone
  email: mail
  iscontact: iscontact
  isactive: isactive
  member_id: member_id
  user_id: user_id
  field_sel_initial_credit: credit &lt;----------
  owner_id:
    plugin: default_value
    default_value: 1
destination:
  plugin: 'entity:person'
migration_dependencies:
  required: {  }
  optional: {  }