What are the specs for the Aprilaire 600 humidity sensor?
The idea that the sensor was based on capacitance is true, but the idea to &quot;design a circuit modification&quot; will not work, because the output of the sensor is digital.
The sensor is from Sensirion and is known as a SHT11:


The SHT1x digital humidity sensor ... As with every other Sensirion sensor type from the SHTxx humidity sensor family, they are fully calibrated and provide a digital output.
The SHT1x humidity sensor series (including SHT10, SHT11, and SHT15) is Sensirion’s family of surface-mountable relative humidity sensors. The humidity sensors integrate sensor elements plus signal processing on a tiny footprint and provide a fully calibrated digital output. A unique capacitive sensor element is used for measuring relative humidity, while temperature is measured by a band-gap sensor. Its CMOSens® technology guarantees excellent reliability and long-term stability. The humidity sensors are seamlessly coupled to a 14-bit-analog-to-digital converter and a serial interface circuit. This results in superior signal quality, a fast response time, and insensitivity to external disturbances (EMC).

It appears that although the chip is calibrated during manufacturing, there is no possibility of changing the sensor's calibration because it's done in one time programmable memory.

The calibration coefficients are programmed into an OTP memory on the chip. These coefficients are used to internally calibrate the signals from the sensors. The two-wire serial interface and internal voltage regulation allows for easy and fast system integration.

There are published projects that use sensors like this one, and it would probably be possible to connect an additional processor, such as an Arduino, then send a serial signal to the humidity controller that followed the protocol of the chip, but really was coming from a program which altered the digital message.  This seems like an awful lot of effort, though.
I suspect that the specs listed above are enough, I haven't covered the serial protocol, but generally, the specifications for the sensor is two wire digital serial communication.
Reference: Sensirion SHT1X Reference Page