In the case of ports configured as analog outputs, any adjacent pair of ports can be used to form a differential ADC. Furthermore, any port of such a differential ADC can be internally biased to form a pseudo-differential ADC. (Multiple ADCs can share a common bias point.)
don't you mean ports configured as analog inputs, not outputs?
(Kind of hard to make an ADC with an analog output.)
Simple RC anti-aliasing on the ADC channels may be all that is needed in many cases. It's true if the system is reconfigurd 'on-the-fly' then the user would eed to know where the ADC input channels are if they are deploying anti aliasing filters. You could get creative with the internal switch to enable 'R-C' filtering on an as needed basis...ut this may not be the best use of Pixi pins. product details and datasheet can be found here....http://www.maximintegrated.com/en/products/analog/data-converters/analog-to-digital-converters/MAX11300.html