The datasheet is a bit misleading here: it describes the voltage limits you've quoted as "absolute", but it's as opposed to differential rather than the safety limits of the device.
The safety limits are provided on page 13, and we see that the analog inputs can tolerate up to 0.3 V below ground, and can tolerate an injection current of up to 10 mA. This almost certainly means there are internal clamping diodes, so you only need to add a current-limiting resistor between the external input and the ADC.
The larger the resistor the better, but at least 15 V / 10 mA = 1.5 kΩ is necessary. Keep in mind, though, that the resistor effectively adds to the impedance of your source, so bandwidth will be affected.
Perhaps more important than the bandwidth, the ADC inputs have a leakage current, which will be multiplied by the source impedance to produce a voltage error. Since the ADC is differential, the error is based on the difference between the leakage currents, typically 1.5 nA in full-power mode. With 1.5 kΩ, this produces an error of over 2 microvolts.