Importing SPCs
From Chrono Compendium
- Grab the Offsets Guide from Geiger's Crypt
- Place your imported SPC in an UNUSED SPACE area
- Change an SPC pointer to point to the new song
- Change the instruments to what the new song uses
...and there might be something else.
Geiger made a patch "Spooky West" (also on the same link above) that you might want to study the differences of to get an idea of how to do it.