To superimpose ripple on a dc voltage use sine wave generator with adjustable amplitude or variable transformer and connect it through a capacitor(≈100µF) to the dc line.
To increase efficiency use an inductor (≈1mH, depending on the frequency of the ac voltage) on the dc line.
Ripple will be present on one side of inductor.
The idea is to stop ripple leaking to the dc supply. Ideal voltage supply will have almost 0 output impedance so if you try to superimpose another signal on dc line it will be lost in the voltage source. By adding an inductor you increase ac impedance of the line so you will be able to add ripple at the other side of the inductor ..