Well. First of all you will need a plugin that will load multiple worlds.
I am not a expert with multiworlds so use mine as a last resort.
I would then create a new using that plugin with the same name of the world you wish to import. Then on your old server, if its not a bukkit based server (ie. Spigot, paper, etc) then go into the folder named "world" from there copy over the folder with the world name. Put it in the place where the new world was created (World/) and replace all the files.
It would be better if you delete the world you created using the plugin before transfering your old world over.
This way of transfering worlds means you shouldnt need to deal with the sponge world properties folder.
If you are transfering the world from bukkit then the world folder is right in the root. Its not in the world folder