I don't use HSPICE, but the Synopsys website says that HSPICE supports transient noise analysis. I would think you can simulate noise performance of a switch cap circuit using transient noise.

If the circuit is continuous time switch cap then I think the simulation is relatively straight forward. If you have discrete time outputs, then you have to be careful that you look at a sampled output.

There is a good tutorial article on sampled noise analysis and simulation in the latest IEEE Solid-State Circuits Magazine.