You connect a bunch of AC sources in series (if you change the frequency, magnitude, and delay of each one and have enough it can approximate other non-sinusoid signals using Fourier). It just superimposes each AC signal onto another. Anways, then you connect a DC source in series with the AC source. The DC source is your offset.
Put a large capacitor in series with the source, and you will find that the AC+DC sources are superimposed on one side of the capacitor, but only the AC appears on the other side of the capacitor (if the capacitor is large or if the AC source frequencies being used are high enough).
I forget the code to do it, but that's how you do it (maybe it's GUI now).