I followed the Sound on Sound Synth Secrets series to build it, but something tells me it's just a fancy subsynth. Some one please confirm or deny: Sum Noise (under Oscillator tag) is indeed Additive in the technical sense of the term if set to all sines.
I believe additive synthesis is usually considered building a tone using sine waves at the fundamental and harmonic frequencies. You'd have to adjust your coarse adjustment for each sine osc.. the first one to 0, the second to 1, the third to 1.5849, the fourth to 2.
If you did this, you'd be able to fake a square wave-ish by setting the coarse values to 0, 1.5849, 2.3219, 2.80735, then by making sure the power for each osc was linearly decreasing by some amount.
@afta8 built a SineBank32 patch which I can't find in the forums anywhere, and it used black calculus magic to provide a bank of 32 sine waves (each at the fundamental or nth harmonic) pretty efficiently.
Here's a 16 sine bank that the uses the black math alluded to by @jjthrash. It can easily be extended to 32 harmonics if you're a dog with good tweeters.