Hi Giuseppe, good question but could you provide us with a bit more information? The basic answer I'm planning to give (which I can give in more detail soon) is that you can construct a creation operator as an MPO and use that.
But could you be more specific about the initial state? As you know, states with well-defined momentum have an ill-defined position, so it's not exactly possible to put them at a large distance apart and have precisely specified momenta. So perhaps you are wanted to make wave packets such as Gaussians which are peaked in both momentum and position space?