Can I leave the input pins of an ADC channel in floating state?
You should tie the pins to a known potential. From section 3.1 of the datasheet:

3.1 Analog Inputs
(CHn+, CHn-) CHn+ and CHn- are differential input pins for channel n. The user can also connect CHn- pin to VSS for a single-ended operation. See Figure 6-4 for differential and single-ended connection examples.
The maximum voltage range on each differential input pin is from VSS-0.3V to VDD+0.3V. Any voltage below or above this range will cause leakage currents through the Electrostatic Discharge (ESD) diodes at the input pins. This ESD current can cause unexpected performance of the device. The input voltage at the input pins should be within the specified operating range defined in Section 1.0 “Electrical Characteristics” and Section 4.0 “Description of Device Operation”.

Italic section is my emphasis. Leaving the pins floating implies that the potential could float to other ranges. Best to tie them to VSS or VDD to ensure they stay at a known level, and within the maximum range specified on the datasheet.