A voltage source placed in the output of a DC-converter?
This type of boost circuit transfers a fixed energy per cycle (determined by the duty cycle and input voltage). Taking into account operating frequency, energy per cycle x frequency = power.

In other words the converter transfers power - it knows nothing about voltage regulation so, if there is a voltage source in series with the load (as shown on your diagram) it matters little - it will try and push an amount of power into the resistor and this power is dependant on input voltage and duty cycle.

So, assuming the power delivered to the resistor is just the right amount to keep the voltage constant across the resistor (Vr), increasing V2 from zero to (say) 30V means the output increases from Vr to Vr + 30.

Just in case you didn't know, this type of booster requires a control system forming a feedback loop in order to keep voltage constant. Without that control loop, theoretically the voltage will rise really high on no-load and break down the diode, the FET and / or the capacitor.