Right...
The OP meant well, but his question was "how to change the impedance of a speaker". the real question is how to use High Range, and Low Range drivers together, in a system, without causing an impedance mismatch seen by the amplifier. Because, he is right... without a crossover at some range, the impedance will be too low. Obviously in a properly designed crossover there will be a distributed impedance across the entire frequency range that will provide corrected impedance over that range, and at the very important crossover region of the two drivers, seen by the amplifier. Determining the correct Xover frequency for whatever those two chosen drivers are, becomes critical. -3dB there, for each driver. Then the frequency response of the individual drivers is left. There is always the possibility that the chosen drivers do not even cross at a desirable frequency, or level. Driver mis-match.
In a pinch, manufacturing specs could be used, but safer to measure each driver, and both drivers together, properly crossed over in the chosen cabinet, with pink noise, using an RTA with calibrated Mic, to see the acoustic response over the range. Your handy anechoic chamber will help as well.
Using an electronic Xover is another easy way to manually adjust the Xover region, and levels (using two amplifiers) as you watch the response. Then, with that information, you can design the right passive crossover for the job, or just set your electronic one, and be done.