Adding spiciness to indian curry at the end of cooking
You probably can't fix the sauce, but the common ways to add heat/spice/flavor to a S. Asian dish after it's cooked are to mix in a tempering oil and to use spicy pickles. The former's easy -- heat up oil and spices in a pan until fragrant, then stir into the dish. You're cooking the spices at the temperature of the hot oil, which is much higher than the temperature a water-based sauce can attain. The latter's easy too -- buy some nice spicy lime pickle at the Indian grocery store!