Step-Down/Buck-Converter: How to replace feedback voltage reistor divider to electronically control the regulator output voltage
Unfortunately I can't elaborate on how to determine the resistor values because I don't remember but there is 1 or 2 places online that at least did tell you the equations.

But what you can do(if I understand what you are asking for) is to simply tie a 3rd resistor to the feedback pin and then drive the other end of that resistor with a voltage output DAC.

The 3rd resistor can also be omitted if you use a current output DAC, the point of it all is that by ether injecting a small current into or extracting a small current out of the node tied to the feedback pin you can force the voltage regulator to output any voltage you want(within it's range of course) even voltages below the reference voltage which usually is the minimum voltage that the regulator can be adjusted to.

If this is a viable solution to what you want to do please say so in the comments to this post and I will try to find the examples of when I used this(I am quite sure I can find out how to do this but it is such a labour intensive task to find those papers that I want to be sure that is is what you want before I go on a rampage through all my old stuff, I'd be really happy to do it if this is a answer to what you want to do), and if it is I will then update this answer to include a solution for how to attain the appropriate resistor values.

Honestly I am sort of ashamed that I still can't work this out without looking up my old papers for equations I have received from someone, using Kirchhoff's Current Law(KCL) and Kirchhoff's Voltage Law(KVL) this shouldn't be a big deal to figure out, but I can't utilize those to understand how. Which I my self really should be able to do by this point in my electronics endeavours, though this is more a solution that real electronics engineers utilize.