Hello, Shourya.
Fyodor, the NMap author, is certainly a clever man, but I'm afraid he hasn't figured out how to code a psychic mode onto NMap.
With that said, you can logically deduce that there are machines behind a firewall by deducing information from open ports on the firewall itself, but enumerating and scanning the separate protected machines, I'm afraid, is impossible without a far more nefarious attack than NMap.
