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In the level parameters I added a new parameter and named it "Custom_Shader", then in the text box next to it called it #SNOWY (I believe that was the name at least). There's other ones you can use too, such as #SKY_ARK which adds low lying fog and wets the floor a little.
Then for the snow particles I used another custom parameter called something like "GPU_PARTICLE_FIELD" which in the text box I wrote #SNOW_EVERYWHERE
This is all from memory, but as I said you can check in the editor yourself, because I haven't used it in a while.
What build are you using?