Be aware that the "digital" step/dir signals provided to modern drives are a stream of pulses, each pulse representing a demand for a move by a defined increment. I don't know much about the 7i33 but on the face of it, a "D to A" converter wouldn't be able to convert a pulse stream to an analog 0-10V voltage, so the scheme you suggest doesn't sound workable to me.
Perhaps starting with a 7i77 and the existing analog servo amps, then moving to the 7i76 with step/dir drives would work as you suggest. From what I know you could swap drives over but you'd want to check with someone who knows.
