Well, not knowing what the crossover points are in the system is going to be problem. You can't assume that they are the frequencies the Focal set is designed for. Chances are, they will work, but they not be optimal, not normally the way one goes about adding expensive 3-way expensive drivers like those Focal units. You do not use the passive crossovers supplied with the Focal components if the amplifier does the crossover work already. If you could find out what the crossover is for all three OEM leads, that's where I would start.
Or consider getting a line out converter and installing a 6 channel aftermarket amplifier, like this:
LOC:
The LC2i, is a compact, two-channel processor with active input circuitry, which allows it to accept speaker-level signals of up to 400 watts per channel from factory-installed source units or amplifiers.
And amp
ONE 60.6 - Mosconi 6-Channel 65W RMS x 6 One Line Series Class AB Amplifier. Last but not least high grade Analog Device ICs have been used in the input signal interface circuitry. 6-Channel Amplifier.
or
Find many great new & used options and get the best deals for Alpine PXE-C60-60 6-Channel Hi-Res Amplifier with Automatic Sound Tuning at the best online prices at
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Or this amp which doesn't need the LOC:
Class-D Full Range 2-Ohm Stable 6-Channel Amplifier. 3-way protection circuitry (thermal, overload, and speaker short protection). Variable low-pass filter: 500 - 5,000 Hz. Variable high-pass filter: 30 - 300 Hz.