How do you insert a Minecraft landscape in an old world?

How to add a Minecraft landscape zmb. Mountains in one world?

Are there mods if any?

Worldedit
if you have worldedit take a wooden ax in creative mode
and select the area you want to have in the other world by left-clicking and right-clicking
// copy
// schematic save (name)
then when you are in the other world easy
// schematic load (name)
//paste
you have to position yourself correctly so that it is inserted where you want it to be.
if it's not the way you wanted it to be, just undo it with // undo.

but there are also tutorials on youtube