Can I use an ADC pin as AREF?
Are you familar with what Aref even is? Either I misunderstand your question or we should explain the basics to begin with.

An ADC is comparing the input voltage with a reference voltage and gives the proportion of this as a digital output:
$$ADC-Code = \frac{V_{in}}{V_{ref}} * 2^{n-1} $$ , with n beeing the resolution of the ADC in bits.

This reference voltage is supplied to the ADC inside the Arduino-µC by the Aref pin. In your schematic it is connected to 3.3V, which is totaly fine. It means, that the maximum code your ADC can give you as digital output corresponds to an input voltage of 3.3V.
Your thermistors build a voltage divider with the \$10k\Omega\$ resistors. This voltage divider is also fed by the same voltage as the analog reference voltage. This makes sense, because noise on the supply voltage will be present in the signal as well as in the reference - so the ADC conversion will not be affected by that noise. If you have a clean power supply or you don't measure with high resolution this is not absolutely necessary. 

Now, to finally directly answer your question:
NO, it is not possible to use one ADC inputs as Aref. The inputs are only signal inputs, you are not able to supply the ADC via these pins. If there is no extra Aref pin, the ADC is simply supplied via the normal Vin pin (it either uses the full supply voltage as reference or has an internal reference, that generates something like 2.048V. This will be possible to select with a register of the controller). 

So, just connect the top of your voltage divider to the supply voltage as well and make sure you have a clean power supply with good decoupling - this will minimize the noise you see in your conversions.