Mapping roles to users while migrating
You can migrate users with roles without migrating roles using migrate static_map

In migrate set 

  process:
  uid: null
  name: name
  pass: pass
  mail: mail
  status: status
  created: created
  access: access
  login: login
  roles:
    plugin: static_map
    bypass: true
    source: roles
    map:
      5: new_role_name
      3: editor


and in the source plugin you have to set the roles in prepareRow

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    $uid = $row-&gt;getSourceProperty('uid');

    $query = $this-&gt;select('users_roles', 'r');
    $query-&gt;fields('r', ['rid']);
    $query-&gt;condition('r.uid', $uid, '=');
    $record = $query-&gt;execute()-&gt;fetchAllKeyed();
    $row-&gt;setSourceProperty('roles', array_keys($record));

    return parent::prepareRow($row);
  }