I think this is not going to be at all possible, remember each set of these IEMs is tuned for 'room response' in an artificial head, each EQ and crossover would be slightly different and you would need inside knowledge of this process in order to replicate it for your own head.
all the same, while not as simple as you say, it would be 'possible' to make your own 6 channel DAC, with digital XO and tune to your own taste, I think the idea of using a simple passive EQ is doable too, but I cant see the results as being very desirable; unless you had it in the connector (an issue in itself) using SMD caps and R's. if you had it anywhere else, each signal would have a long distance between the driver and the XO, phase distortions galore