Yes, you could use a separate generator, with a shunt regulator. But why? A bike dynamo, properly 'managed,' DOES supply enough amp-hours to light the way for you, as does a "white wire" system. The trick is using the available power - in managing it efficiently.
An unmanaged bike dynamo really has nearly zero power to waste. However, we should also keep in mind that our
motors don't have a much power to waste, either. A 50-70 watt dynamo, equivalent to a typical motorcycle output, would suck up 5 to 10 percent of the total engine output...
No matter what we use, it has to be fairly efficient. A shunt regulator works by shunting the excess power through a big resistor to ground, and the extra load causes the generator voltage to drop to a point which won't overcharge the battery (in effect, going into a 'trickle charge' mode.) However, this wastes the excess power as heat. But, even a motorcycle, with an undersized (compared to an auto) generator, has watts to throw away, as compared to a bicycle engine.
However, the focus of this thread is use the power that
is available in a bike dynamo efficiently, for our lighting, as opposed to slapping a separate generator on the bike, and, in the process, wasting a noticeable fraction of our total engine power.
Keep this in mind, too. A bike dynamo is rated at 3 W. This rated power output is at a speed of 12-15 km per hour bicycle speed, or at about 10 miles per hour. At 20 MPH, a bike dynamo produces about 6 watts. Although STILL not a lot of power, it DOES give us more leeway in the design. So, if I can average 20 MPH on a 10 mile trip, and am using efficient LED lighting, rated at 3 watts, I will end up with an 'overage' of 1.5 watt-hours for the trip. That extra 1.5 WH is more than enough to power the regulator circuitry, and to maintain the battery.