Dragon' Den in the SNES version

It's possible to add new maps with a custom build of FF6LE like FF6LE Rogue CE. It allows NPC, event triggers, exits and maps expansions. For monster, it's possible to expand them as you can see in this thread but it has to be done manually. What you could do is relocate the data and then edit your monsters with a vanilla ROM and FF3usME to finally transfer your changes into your ROM with a hex editor. As for monster GFX, the latest source of FF3SE can be build to allow monster GFX expansion.

So in one sentence, it's technically possible to add the Dragon's Den to the SNES version.