The legacy mode could be easily implemented by making sure both sets of graphic and audio files exist in your project, and use a boolean variable to toggle the 2 types. You would just then need an 'If boolean = legacy' then use gfx set 1 and audio set 1.
You can easily try some prototyping using the free version.
Using .SPC (is it SNES audio?) would require a plugin to be created, similar in principle to what Gigatron did for amiga audio - LINK here if interested.
Otherwise, you would need to convert them yourself.