Essential fatty acids are...well, essential...for everyone regardless of doing a colon cleanse; particularly omega 3s since our diets are usually imbalanced with omega 6s. Flax seed/oil is one way of consuming omega 3s, but I have read some research that it indicates that many people are unable to break it down into a useable form of omega 3s and also have difficulty tolerating it. Dr. Mercola talks at length about flax seed oil vs. fish oils on his site...mercola.com. There are also beneficial components of flax seeds that are not found in the oil...fiber and lignans. Where flax is concerned I recommend using the ground seeds instead of the oil...they are quite tasty and make a nice addition when sprinkled on cereals, salads, soups, yogurt, etc. I personally use fish oil for EFAs.
Whatever oil you use, I wouldn't add it to the psyllium/bentonite mixture...if that's what you mean by supplementing it. Any supplements, herbs, medications etc. should be taken at least an hour before or after the p&b shakes.