According to the datasheet for the MCP3424 the differential input impedance is 2.25/PGA MΩ. For an input voltage of -2 to +2V the PGA would be set to 1 so the impedance would be 2.25 MΩ.
To increase the input impedance you will need to put an operational amplifier in between your sensor and the ADC input. Op amps have an input impedance typically between 1 MΩ to 10 TΩ so you should be able to find one with an impedance of more than 10MΩ.
Before adding op amps to your circuit I would recommend checking the stability of your power supply for the sensors and the Raspberry Pi as noise or instability on the power supply could cause you to get unstable values from the ADC inputs.