My guess is, download the world, put the files of your world in the newly made dimension within the world and upload it again
Just tried this and it worked! Thank you for the help! 
Posts by aaqquuor
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This is intended.
Magma handles multiple worlds as dimensions like Vanilla, so they are part of the main world.
So is there a way then to upload custom maps to other multiverse worlds? Without FTM of course.
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So I've used multiverse before, the most recent version playing with my friends. We decided we wanted to add mods too so I went to magma and used that. I added the mods and multiverse and it went completely fine, I created the worlds in game and even went in it, made sure it wasn't unloaded and all that.
But whenever I go to my worlds tab, none of it would appear, not even the nether nor the end. I don't know if this is normal or a glitch but help would be appreciated!