Well, you CAN use a resistor, but don't be particularly surprised if it doesn't work as well.
There's three applications where I've put a resistor in series with a Faulhaber, and many more that I've put resistors in series with 3.5v. tiny gearhead motors.
The installations with the robotshop Solarbotics gearhead motor I call the "M&M" motor
http://www.robotshop.com/ca/en/solarbotics-gm15-gear-motor.html have been extraordinarly successful repowering Kato 11-105 chassis for tiny geared steam locomotives. You get incredible slow speed and wonderful torque. Because of the gearhead, the current is relatively constant, so the resistor doesn't really take that much of a hit in series. While they are incredibly slow, they do have a noticeable whine at speed, just sayin'. If I do a powered hirail truck here, that's the powerplant.
Putting a resistor in series with the original Kato 3.5v. motor and no gearhead is a disaster, that's why they changed it over to a 12v motor in all current production although the traces of the original design remain with the little circuit board to put the resistor on. Bad idea. Hot enough in service to melt plastic.
I've put a resistor in series on my scratchbuilt 44-tonner, and later my 28-ton shay (no room for a gearhead) with a Faulhaber 0816, and while it ran 'slow enough' and was incredibly quiet, if you put it on a grade and loaded it up it was darn near uncontrollable as the resistance/voltage equation shifted under load. I recently replaced it with a 3.5v gearhead in the Shay and same resistor and was satisfied with the results.
The 30-ton Climax B has a 0816 Faulhaber in it, and a resistor, no gearhead. I have a worm right off the motor shaft to drive the jackshaft, it has to be a good stiff shaft without support, and there's no room for a gearhead. Because of the second worm, replacing it with a plastic-shafted gearhead motor won't work either. So the Faulhaber with a resistor and no gearhead is still in there. It's OK, and that mechanism is so tricky that I'm not about to mess with it any more. Done.
My favorite power plant has become a hybrid - take a Kato 12v motor and "D"-grind the end shaft, put it on to a Gizmozone plastic gearhead (normally on a 3.5v motor same case as the Kato) with a 5.14:1 reduction, and a universal. That's a cheap and sweet combination, even if you only get the Kato chassis for the motor and the Gizmozone for the gearhead. Dead quiet, right speed reduction, killer torque, and it's ideal for repowering Kato chassis, all manner of tender drive small steam, and single-trucked Tomytec drives like my Whitcomb switcher kit. No resistor, tiny high-torque application, inexpensive, and much, much smaller than a Faulhaber.
To those that say the gearheads won't hold up I have a simple answer, these little guys are so expensive you simply get spares. In seven years of messing with these I've only really worn one out where the motor got bad and I just swapped one out. I've pretty much standardized on the M&M drive for slow geared steam like Shays and my Climax A's and the Kato+Gizmoszone head for anything that has to move at 30mph normal running speed.
I've also done head-to-head performance tests on a 'stock' Tomytec chassis with a 12v motor and a brass flywheel vs. one of my Kato+Gizmoszone hybrids and no flywheel, and no doubt about it, the non-flywheel is far easier to switch with, better speed range, just as quiet. The gearhead is better than the flywheel in terms of performance impact.
That Kato 12v motor case is a lot more standard than I ever imagined. That case design and dimension is identical with the double-shafted design on the Bachmann 44-tonner motor, and is indistinguishable with the 3.5v motors that preceded it. I have to mark the motors to tell them apart. The case design on the Atlas 4-4-0 is the same as well, so tinkering with them is worth it. I've always wanted to put TWO Gizmoszone hybrids on the end of a Bachmann motor for a double-trucked variant, it should work just fine as the case is drilled for it.
