What's so hard about making a simple emitter that just spawns particles across a large area which just fall down at constant speed?
Try setting up a simple sprite emitter with high particle count. Initial location should be a box with Z=0 and X,Y=+/-(size of area with falling snow). Experiment with the (Initial)ParticlesPerSecond and MaxParticles values (make sure both PPS values are the same) and make sure that ParticlesPerSecond * LifetimeRange.Max is at most equal to MaxParticles. Set the particles to never respawn and disable automatic initial spawning.
Choose a LifetimeRange that, in combination with the initial downward velocity, makes the snow particles barely reach the lowest point on the ground.
I recommend a relative warmup time of 1.0 and warmup ticks around 10. To make your snow collide with your level geometry, enable UseCollision and UseMaxCollisions. The MaxCollisions range should be set to (0,0) so particles are removed when they collide with the level geometry.
An emitter set up this way might be quite a performance hit, you should try to split it up into several emitters with smaller spawn location areas. (And I really mean several emitter actors, not just several particle emitters within the same emitter actor.) Try to make their spawn areas fit to the area of the map they are supposed to cover. The rectangular nature of the spawn location boxes should make it relatively easy to align them properly.
