Hmm are the subs passive and external cabinets and you want to pull from the crossover out run another crossover and have an external wire to the sub?
This has several problems:
-The power rating of the crossover. This will draw mich more than originally intended and at higher levels could smoke the crossover.
-I'm not sure the quality would be idea when running a crossover and another filter, but I could be wrong.
-I've never seen sucessful integration of a speaker level sub as such
-in my opinion, Im not sure this would be better than what you have already. Some amped subs can be had for so cheap anyway then you could high pass it from the receiver.