Figured it out. It does support addModulator. This is how you do it:
d3660d2c-72ae-45d4-8e6d-f5110a221950-image.png
Content.makeFrontInterface(600, 600); const var builder = Synth.createBuilder(); builder.clear(); const var generator = builder.create(builder.Effects.SlotFX, "generator", 0, builder.ChainIndexes.FX); const var slotfx = Synth.getSlotFX("generator"); slotfx.setEffect("Hardcoded Master FX"); const hardcoded = Synth.getEffect("generator_Hardcoded Master FX"); const numP = 4; for (i = 0; i < numP; i++) { hardcoded.addModulator(i, "MatrixModulator", "P" + (i+1) + " Modulation"); } builder.flush();