If you have a .7z file and want an .nds file, you don't actually need a "converter." A .7z file is just a compressed folder (like a .zip) that likely has your Nintendo DS game hidden inside. Here is how to get your game file out: How to "Convert" (Extract) 7z to NDS

Remember: You cannot convert a box into a video game cartridge. You open the box to get the cartridge out. Download 7-Zip, extract your file, and enjoy your DS games safely.

• MIME and signature-based detection; heuristics to map extracted files into NDS partitions (e.g., allocate .nds/.sav/.dsi files appropriately).

Before extracting hundreds of ROMs, check if your emulator already handles 7z natively. If it does, you don't need to do anything at all.