U can board !!
Subject to availability in any class.
Simply purchase a first class unreserved ticket @ Panvel and talk to TTE of this train @ Panvel, when it arrives there, for availability of seats. If available, board the train, TTE will collect the difference in fare, to make it a full fare of particular class from Ernakulam to Nizamuddin. He will give u a receipt and allot u seat.