There's actually a way to do this without the use of any ASM. It's a technique that I came up with myself. All it requires is Lunar Magic and a GFX editor like YY-CHR. I'm assuming that you know how to use a GFX editor. If you don't, there are tutorials that can teach you.
Believe it or not, there are a bunch of 8x8 tiles on the overworld that are either duplicates of one another, or completely unused.
Examples of duplicate tiles (red circles)
Example of unused tile (red circle)
After exporting your GFX, find 1 of those 8x8 tiles in your GFX editor and change it into a symbol (e.g. star). Go back into Lunar Magic, import your GFX and go to the Overworld Editor. If any tiles on your OW don't look right, it means that the tile you edited was used somewhere on the OW. Just replace the tiles with one of the duplicates. If that doesn't work, then the tile was probably included in one of the Layer 1 tiles. Go down to the bottom of the OW...
...and replace the glitched tiles with one of the duplicates.
To the right of the OW, you will see your Layer 2 event tiles.
Each event tile is a 16x16 tile, but all 16x16 tiles are made up of 4 8x8 tiles. Make new event tiles with one of the 8x8 tiles as the new tile that you created in your GFX editor. Every time the player completes a level, simply add that event tile to the OW when the level is completed.