Does anybody know what am I looking for?

[Acies]

This thread should probably be placed in developement support plus have a better title. Anyways. It is possible, I would guess. All the particle editor does is to spawn a number of images and then make them move, fade, size, resize etc. in different ways.

The image could be "directly imported", I do not think you should go around importing any .ps files. What you would need for snow:
1. An image of some snow
2. Replace the material/image of the "rain particle system" with your snow image and then save it.

As for your second question I do not know, since I do not remember/know the effect you are talking of.

Good luck