  We present a new method to add long wavelength power to an evolved $N$-body
simulation, making use of the Zel'dovich (1970) approximation to change
positions and velocities of particles. We describe the theoretical framework of
our technique and apply it to a P$^3$M cosmological simulation performed on a
cube of $100$ Mpc on a side, obtaining a new ``simulation'' of $800$ Mpc on a
side. We study the effect of the power added by long waves by mean of several
statistics of the density and velocity field, and suggest possible applications
of our method to the study of the large-scale structure of the universe.
