The modulators in the Modular Subtractive synth operate at a control rate that is slower than audio rate for performance reasons.
But more than that, an FM synth really demands a very different architecture than a traditional Subtractive Synth, so I would envision it as a completely separate synth component. With that said, just as a test, I was able to implement FM modulation in a custom Synth Component–I totally recommend playing around with that if you feel comfortable in C++!