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:
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And amp
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or
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Or this amp which doesn't need the LOC:
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