Like if I create a regular Minecraft world, can I open it in world painter and edit like a mountain into it? Because I'm making a 2k survival mountain map in WorldPainter, but then I thought that it would lack a bunch of resources like stuff you find in deserts, plains, jungles etc. So I thought it's be great if I could make my 2k map a part of an 'infinite' Minecraft generated world. That way I could play in my map, but travel for resources etc. Is it possible?
Actually, yes you can. WorldPainter can import Minecraft maps (File -> Import new World -> From Minecraft map...), and after editing the world you can merge the result with the existing map (File -> Merge with Minecraft map...). It's limited, you can basically only edit the type and height of the surface and add things on the surface (WorldPainter is still a map generator in principle, not a map editor), but you can do things like you said.