I'm surprised no one's suggested this. Tuning, and your room's response.
But FIRST, I would see if the drums need to be re-tuned. Do they sound "fat" already, in your room? How do they sound in your room? You can EQ to your heart's delight, but if it ain't there to begin with, you can do more damage than good.
Most of the time, drum issues are often caused by bad acoustics, improper tuning, poor playing skills, mic-technique--- in this order. So check along these lines first.
What mics are you using?
What exactly do you mean by "fatness"? Thick-sounding? "tuby sounding" ? "Power/punch"??
Usually "fatness" falls in the area of 200-800Khz. Once you've worked through the initial trouble-shooting I outlined above, then you can try subtractive EQ treatment.
Best of luck!