Being that speakers run on AC power, the resistor would have to be non-polarized. It would be quite difficult, and probably rather expensive, to find a resistor with that low of an impedance and able to handle that much power. You'd be better off to create a coil out of some solid 32 gauge wire to get to the resistance you want, but then you would have to keep it cool. You'd also be losing power there anyway getting the sub itself only half of the power from the 2 ohm load, thus making it useless anyway.
Best solution, sell the sub, buy one with the coils you need.
They use non-polarized capacitors for those. Different farad rating yield different crossover points, however the final ohm load of the driver can change all parameters too.