How to migrate multi role to an user using migrate csv plugin
First put your all roles inside double quote separating by comma (,) and be sure that your all roles on csv file must be role's machine name.



then edit your config file something like this....

process: 
  name: name
  mail: email
  pass: password
  roles:
    plugin: explode
    delimiter: ','
    source: roles
  status: status
destination:
  plugin: 'entity:user'
  md5_passwords: true


you have to use plugin: explode provided by drupal to import multiple roles separated by comma (,).

And if you are importing password also then use md5_passwords: true