Modifying LM2596 circuit with AD5206
The main problem you will have is the voltage that the AD5206 will see. It is only rated for voltages on any of the pot connections that are equal to (max) or within the range Vdd to GND. I suspect that the circuit diagram for the LM2596 module will be something like this: -

 

Note the position of the 10k POT in the circuit. Basically it's connected to the output terminal and although you indicate 680R in series with it, if you "dial-up" an output voltage that is too high the pot terminals may go beyond the absolute maximum ratings specified in the data sheet.

If you can guarantee that the POT won't go beyond Vdd to ground you then have the problem of the POT's capacitance to ground. If you read the data sheet there is typically 60pF from each wiper to ground (internal) and this could cause all sorts of problems with stability on the regulator (and noise of course).

What I would recommend is that you try the circuit with an external regular pot and add the parasitic components specified in the POT's data sheet and see if the circuit goes unstable.