Imposing AC ripple on DC voltage
For simulation, just connect the two voltage sources in series. You can do the same with the AC source directly, as vangelo says, but it's more obvious on the schematic if you do it with two voltage sources. 

For a real circuit you can do the same if at least one of your voltage sources is not grounded. 

If the voltage sources are grounded, then you could add a series inductance (a few, say 3H) in place of the diode and something like 1000uF/50V electrolytic low-impedance blocking capacitor in series with the AC source, however the AC source needs to be protected against the 36V surge at power-on and capable of supplying the load current plus reactive current through the 3H inductor.