Is it possible to read DHT22 through I2C ADC (i.e. PCF8591)?
Not in any useful sense. The DHT22 communicates digitally, using a protocol described in the datasheet. It doesn't output an analog temperature signal; if you want that, use another part, like the LM35.

You can't use an ADC to read the DHT22 because:


It's not an analog signal. (I mean, you could use an ADC to read a digital signal, but it'd be a huge waste.)
The DHT22 doesn't start outputting its status until it receives a pulse from the host MCU. If you hooked it up to an ADC, it'd just sit there and not do anything.