Sure, set your emitter's spray cone to 180 and the Y randomiser to the height of your screen. Place the emitter just off the screen in the centre. Be sure to set the particle life timeout enough to make its way across the screen
Just a warning, from another 3D Max user: don't expect much control in terms of the particle systems. You can't spin or manipulate the sprites themselves in much other than size and opacity (obviously speed and so on as well, from the emitter).
I suggest that you render a few quick animations of spinning snowflakes and use create a sprite sheet for each. Then use multiple particle emitters, each with a different sprite sheet, to create some variety.
EDIT: Damn, wait. I just realised that you can't use animated images for particles. So scratch that idea altogether :(
EDIT 2: Again, damn. I just saw now that this is Construct Classic. My apologies. I've got no idea if my first suggestion will work, if those options are even there. The capx certainly won't work :P