There's many ways this can be accomplished, but here's a method that relies entirely on free software. By the way, Steps 5-9 work for basically any .wav sample you want to make an instrument out of.
1) Find a song in .vgm/.vgz format on
http://vgmrips.net/ and dump it using vgm2opm or vgm2pre (the original sites are down: you have to hunt down mirrors of these software)
2) Download and install OpenMPT
3) If you dumped a .opm from your .vgm/.vgz, get the VOPM VST and hook it up to OpenMPT.
3b) If you have .eif/.y12/.tfi/.tyi/.vgi, additionally download Deflemask Tracker (You still need OpenMPT for later)
4) Whether you used OpenMPT or Deflemask, open your preset you dumped from the .vgm/.vgz with VOPM (if OpenMPT) or the Genesis instrument editor (if Deflemask) and play a fairly long note with the instrument, then export it as a .wav
5) Go to OpenMPT's Samples Tab and create a new Sample, then Drag/Drop (or open) the .wav into the Samples tab interface.
6) Loop the sample, and try to make it as short as reasonably possible, keeping in mind that SPCs have a joint max of 64kb to store all music data including samples.
7) Save the sample from OpenMPT as a .wav
8) Hook up C700 to OpenMPT, then drag/drop your new looped .wav sample over C700.
9) Finally, in C700, export your sample as a .brr, and you should be good to go