Yes, I've been able to use the showers in the lounge upon arriving from JFK even when HEL is my final destination. You can try Hilton Vantaa (walking distance but just outside the airport terminal) or Glo (inside the terminal) and ask for day rates; this will allow you to use the room from 6 a.m. to 6 p.m. You can't book these rooms through the Internet but you can call the hotels directly or just walk in.