Adjusting a fixed-voltage DC/DC converter's output
Yes it's feasible and yes it's commonly done. It's called 'margining' and as you guessed it involves corrupting the feedback signal with some external current to make it believe it's at another level. Analog Devices makes a dedicated DAC but it's expensive and difficult to source. In fact any voltage output DAC can be coerced in service simply putting a resistor between the output and the FB node of the regulator. If the impedences are not known some experimentation with the value would be needed.
There's a somewhat detailed procedure in the DAC43401 datasheet, 9.2.2 power supply margining
Of course it's not a magic technique, you'll also incur in limitation due to the feedback loop amplifier and duty cycle of the converter.