Can one use PiFace for A/D conversion?
There is a small data sheet for the PiFace here on Farnell. There's a schematic linked to from this post.

It has digital inputs and outputs only, no ADC. Most sites advertise it using 'PiFace Digital I/O Expansion' as its full name, with no mention of analogue. 

You don't need to use PiFace IO expansion to read temperature sensors, the gpio will do, as in this adafruit tutorial on reading a DS18B20.