Is it possible to create new maps for Worms 3d/Siege? Any sites where I can d/l some maps?

1: W3D? Yes. K^2 made one, but it's very hard to use. It's also not on this forum, and I dont want to go searching for it.
2: No place to download. As I just stated, K^2's map editor is very hard to use, and I dont know of any maps created by it.

I make Mod for 20 mission maps for play on multiplayer, On my site you find it. But I only replace old maps on new.

You can do that manually. It's easy. You can also transfer maps from W4M to W3D if you own both games, and vice-versa. It's just a case of renaming and copying a few files.