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();