of course u can play custom maps on an unranked server.
2. Enter your age (if asked for it)
3. Click "See all Downloads"
4. Look for "Crysis 2 Community Dedicated Server"
5. There should be a .pdf file in that download that will help u with custom maps, private server, etc.
P.S.: This only works with an unranked server, but as u´ve got one, there shouldn´t be any problems.