You cannot mix original Nitro Grand Prix tracks with custom tracks in the same cup without manually rebuilding the cup list in a hex editor – but Nitro’s pre-built GP already solves that.
Would you like step-by-step instructions on how to load a custom track GP in MKDS Nitro, or how to build your own cup list? Mario Kart DS- Custom Tracks Grand Prix Nitro -...
It sounds like you're referring to modding, specifically the Custom Tracks Grand Prix feature found in Nitro (a popular MKDS ROM hack or mod pack, often associated with Mario Kart DS Deluxe or Nitro Custom Track Distribution ). You cannot mix original Nitro Grand Prix tracks