A change of speaker impedance; is it possible?

Recently I've been troubled by a heretic idea. In most of the cases the 1-broadband driver speakers (horns etc.) have higher impedance (of 16 or 8 Ohms). The two-way speakers and a large portion of the three-way speakers - 8 Ohms, and the 4-way - usually 4 Ohms. The more the drivers - the lower the impedance. So is it possible to change the impedance of the speakers? With my extremely limited (11 grade) knowledge of physics I would give an answer that is - Yes - by attaching one more resistor of 4 Ohms to the 8-Ohmer and the 8-Ohmer will perform like a 4-Ohmer. However I am sure that many limitations exist. The drivers, the filters, the crossovers etc. are probably designed to work in a specific impedance regime. So is it such an option, of course, without affecting the performance of the speaker?