A file can be opened for writing as a stream using WITH-OPEN-FILE macro.
(with-open-file (file #P"test.file" :direction :output
                                    :if-exists :append
                                    :if-does-not-exist :create)
  (dolist (line '("Foobar" "Barfoo" "Quuxbar"
                  "Barquux" "Quuxfoo" "Fooquux"))
    (write-line line file)))
The same can be done manually with OPEN and CLOSE.
(let ((file (open #P"test.file" :direction :output
                                :if-exists :append
                                :if-does-not-exist :create)))
  (dolist (line '("Foobar" "Barfoo" "Quuxbar"
                  "Barquux" "Quuxfoo" "Fooquux"))
    (write-line line file))
  (close file))