Hello, I haven't a lot of experience of working with NS2. But I know a little bit about networking. At the first it's important what type of Internet Protocol do you wish to use? It's different implementations for IPv4 and IPv6 protocols. Second of them provides a lot of additional space throught extension headers (for example, destination options). IPv4 has more scant header to bring a lot of data. Another way to bring data with any packet is encapsulate raw data into the packet and mark this packet as special. On the other endpoint we may check if packet has spacial mark and fetch data from the packet.
Anyway I need more information about your task. So, it will be easier to select right solution.