Sure, my first thought is to do it in post
One of 2 ways will work fine here:
1. Render out your rain as a separate 3d element so you have more control over the luminance and color (chromanance) of the rain plate in your compositing package.
2. Generate your rain directly IN your compositing package.
Most decent compositors have a particle generator that enables you to achieve rain, etc.
And if they do not, then usualy there are plugins avaiable that can help.
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS