It’s somewhat easy so I wonder why no one posted it before. Just repoint the sound room table (it’s at 0x0CE4D28) to free space and add your new songs. Behold that you’ll have to FF out at least 4 bytes after the last song otherwise the game crashes when entering the sound room.
We should have room for 255 songs but I haven’t tested it yet. (I’ve tested up to 164 songs with the last 14 songs been grayed out for apparently no reason. I still need to test if it’s a hardcoded limit or if it’s just me.)