Can you be more specific about the waveform you would like to generate? There is a difference between a "negative" voltage source, which implies negative polarity, and a "negative sequence" voltage source which implies a certain phase sequence for a three-phase AC power system.
Assuming you want to have a negative sequence voltage source, you can simply construct a set of sinusoids with a different phase rotation. Attached is an example that generates a 1V positive sequence voltage at the 50 Hz fundamental and a 5th harmonic 0.1V negative sequence voltage. You can use the FFT analysis in the scope, with the table configured to show the angle, to observe the phase rotation for each harmonic.
