We're now able to modify the Club Penguin games for the Nintendo DS thanks to a tool called EPFExplorer.
EPFExplorer is a tool for the Club Penguin DS games; Elite Penguin Force and Herbert's Revenge. It was made by Olivercomet.
Current features (as of version v1.3.3):
Opening and editing .ARC archives (Both EPF and HR)
Opening and editing .RDT resource data files (EPF. HR is viewing/exporting only.)
Exporting sprites to PNG sequence, GIF, or (for folders) sprite sheets.
Editing TSB/MPB and NBFC/NBFS/NBFP tiled images. (Both EPF and HR)
Exporting and reimporting Lua scripts.
Editing save files. (Both EPF and HR)
Exporting audio clips to .wav (PCM or ADPCM format wav, openable if FFMPEG is installed)
Exporting music tracks in .XM format, with all sample data
Opening and editing the game's DLC mission (download.arc)
The music replacing also currently has issues with some tracks (e.g. Dojo and Gadget Room, among others)
*you'll have to extract the core files from the ROM using tinke first.
Download it right here:https://github.com/Olivercomet/EPFExplorer/releases/tag/v1.3.3
Here's the very first custom mission in EPF:https://www.youtube.com/watch?v=izkaNKG4ERc