We are interested in the modeling and simulation of the IP networks as the communication layer for smart grids. In this context, a network can involve many different technologies, and the available models corresponding to these technologies may be implemented in different simulation software. As a result, thanks to their own models libraries, different IP network simulators can be complementary. However, these simulators are not all interoperable with each other, and therefore can not be yet all integrated in a same co-simulation. Moreover, the network simulators have to interact with the other simulators corresponding to other areas of expertise involved in the smart grids simulations. Integrating this requires to consider the multi-formalism problems. Our approach is to integrate IP network simulators to the DEVS-based co-simulation platform MECSYCO (formely named AA4MM). Thanks to this approach, different network simulators can exchange simulated IP packets. In this paper, we illustrate how we integrated the NS-3 simulator into MECSYCO.