@Nicola: the two sensors on OS 3.0 are both wired to digital pins so they won’t actually read analog signals. As Mike suggested, you can use an ADC module (for example, something like PCF8591) to read signal from your analog sensor, and hook up the module to the I2C interface on OS 3.0.
There is one ADC pin on OS 3.0 — but that’s already wired to the solenoid current sensor.
OS 2.3 actually has several spare analog pins (those are all named Axx where xx is the pin number). The analog pins can read your analog sensor values.