How do I migrate content to a field added with UI to an existing custom content entity? In a custom module (assoc), I have a custom Content Entity (Person) with these fields:

$fields['lastname'] = BaseFieldDefinition::create('string')
$fields['firstname'] = BaseFieldDefinition::create('string')
$fields['email'] = BaseFieldDefinition::create('email')
$fields['cellphone'] = BaseFieldDefinition::create('telephone')


I migrated Person content from a Drupal 6 site using Migrate Source CSV, Migrate Plus, Migrate Tools and this migration file:  

id: migration_persons
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
  owner_id:
    plugin: default_value
    default_value: 1
destination:
  plugin: 'entity:person'
migration_dependencies:
  required: {}
  optional: {}


I added a field to this entity using the UI:

The corresponding config files are field.storage.person.field_sel_initial_credit.yml and field.field.person.person.field_sel_initial_credit.yml
I tried to migrate some content to this field using  

id: migration_sel_initial_credit
migration_tags: null
migration_group: null
label: 'Sel initial credit migration from CSV'
source:
  plugin: csv
  path: /Users/efalzon/Sites/JdP8M7b/config/migration_Sel_initial_credit.csv
  delimiter: ';'
  enclosure: '"'
  header_row_count: 1
  keys:
    - uid
process:
  bundle: "person"
  entity_id: uid
  revision_id: uid
  field_sel_initial_credit_value: value
destination:
  plugin: 'entity:field_storage_config'
migration_dependencies:
  required: {}
  optional: {}


and I got this error when executing the migration:

[error]  Attempt to create a field storage without a field name. (/Users/efalzon/Sites/JdP8M7b/web/core/modules/field/src/Entity/FieldStorageConfig.php:252)


What am i missing?
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: {  }