If I add an extension tube or two to my lens, will more of the light make it from the lens to my sensor, or less?
The answer here is much the same as for your other question. As you move the lens away from the sensor, the image circle spreads out over a larger area. The sensor is obvious the same size, so it will see a smaller portion of the image. The field of view as seen by the sensor will therefore decrease, the brightness of the image falling on the sensor will decrease, and the part of the image that does fall on the sensor will be magnified more. To get more of the light projected by the lens to fall on the smaller sensor in your camera, you'd have to move the lens closer to the sensor, but extension rings move the lens farther away, not closer.
Just reading an answer doesn't always help you to really see how something works, so let me propose the following experiment...
Try this: Tape a 5"x7" index card in the center of an 8.5"x11" sheet of paper, and then tape that paper to a wall. Shine a flashlight at the paper and adjust the distance so that the flashlight's spot just covers the paper. This is a model of the inside of a camera: the index card represents the APS-C sensor, the larger paper represents a full-frame sensor, and the lighted end of the flashlight is the rear surface of the camera lens. Now move the flashlight farther from the wall while continuing to shine it at the paper. Does more of the light from the flashlight fall on the index card, or less?