Assuming the details of the DX7 are similar to some of the PC Yamaha FM synth hardware, you could do things like set the attack/decay of a modulating operator to be very slow, the attack of a carrier to be fairly fast, and subsequent key presses wouldn't necessarily sound the same. And with a big chained algorithm like the DX7 can do, I agree, you couldn't capture the full behavior through a sample.

You could replicate the most common behavior, though.