That's the question. So let's say I want to do a boss rush in my hack. Can I put 9 Areas in the same level? Does it requires ASM hacking?

You can use the area overflow bug to simply go to different level's areas: https://nsmbhd.net/thread/1823-area-overflow-bug-going-to-any-level-of-current-world-area-overflow/

Fully new areas require asm tho as the area table is hardcoded.

Also for a boss rush you'd have to asm hack the bosses to not end the level in some way.
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GitHub - Kuribo64 - YouTube

As long as bowser's bridge button isn't pressed, just put the boss key location off-screen, so Mario can just continue through a pipe or whatever it is you're doing
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Here's my MvL Hacking thread

Consider joining the NSMB DS Hacking Discord Server! Also, be sure to check out the new wiki!

#HakingNoMore