This should work!
$this->addColumn('table_name', 'column_name', 'type AFTER column6');
examples:
$this->addColumn('tbl_posts', 'email', 'VARCHAR(150) AFTER `name` ');$this->addColumn('tbl_posts', 'phone', 'string AFTER `email` ');
$this->addColumn('{{%user}}', 'username', $this->string()->notNull()->unique()->after('id') );