Can I add two signals together with a magnitude ratio between them? Or what is Sum modulation on the 33500B Series waveform generator?

Yes, any two signals can be added together on a single channel. This is done by using the Sum Type in the Modulation menu. In the Modulation menu select modulation type, Sum. Then select any signal to add to another signal. You can also set a magnitude ratio between them. For example, you can add 10% noise to a square wave or create a two-tone signal with sine waves. You can add two signals together on both 1-and 2-channel models. You can also combine the summed outputs on a 2-channel model to create a 4-tone signal.