as you know, the essence of digital FM in MaxMSP is the modulation of the frequency VALUE of an oscillator, with the SIGNAL of another. As your audio feed is already a signal, you'll have to use this as the modulator, send it to a [+~] object, whose other inlet receives a number value, and send the result to an oscillator or other unit generator - i think that should work

Since FM in it's most basic form is simply vibrato with an audio frequency modulator, you could do it really crudely with tapin-tapout. Here is what I mean, there's a bit of an odd issue with it, in that if you listen to preset 1, the modulation isn't quite right, so it sounds like distortion with faster modulation speeds. Not too hard to sort out I would imagine.......

tim's works; mine doesn't