Anyway we can make it snow?

In arctic biomes, the rain is snow.

What about the entire map?

I don't believe this is possible. Correct me if I am wrong anyone, but I think it is biome dependant.

I believe there is a command that you can put in the server cfg to change certain biomes to other biomes. It wouldn't change their appearance, but it would change the weather to be cold/snowy.

possible, but be careful doing that you may influence other things like what spawns in the biomes etc.