It certainly is possible. The simplest way of doing this is to add a rheostat or potentiometer in line with your LEDs in exactly the same way you would add a normal resistor to get the correct voltage and current. As you increase the resistance using the pot, you would be decreasing the current to the led, and adjusting the brightness. The biggest difficulty with this method is finding a potentiometer with the appropriate values. Most pots that you find now are in the 5k and up ranges, when you'd want maybe 100 ohms.
Another (more complicated but more practical to implement) method involves using a potentiometer to control the amount of voltage to a transistor, and using the transistor to limit the current to the LED.
A third method is called Pulse Width Modulation, or PWM for short. It involves sending electricity to the LED in a series of pulses, and by varying the ratio of on time to off time, you can control how bright the LED appears. This is usually what's happening if you see an led that appears to "seperate" into a series of dots if you scan your eyes by it or wave your hand in front of it. It's usually the cheapest way to digitally control the brightness of an LED.
For an excellent example of a light using the potentiometer/transistor setup, look at the skylight, by rigel systems.
http://www.company7.com/rigel/products/skylite.html
Paul
