Hi Dmitri,
5.5V is the maximum voltage that can be applied to the device before damage will occur. This is a process limitation. The ESD cells are slightly different between the analog and digital input pins. You will note that the analog input pins has the VDD+0.3V maximum requirement. Also note that this does not mean that you can apply up to 5.8V if the VDD supply is 5.5V. Any input beyond 5.5V will damage the device.
As to the digital input pins, even though it differs from the analog input structure, current will flow to VDD if the input is over the supply voltage of VDD. In other words, the digital supply pins should be no greater than VDD.
Another device you may be interested in is the ADS1120 that will be releasing in the coming weeks. This device has a number of additional features that may be of benefit to what you are trying to do.
Best regards,
Bob B