Well, maybe. But the brewers I know make their own recipes when they don't like the kits, and just buy what they need to make a good beer instead of "doctoring" an inadequate kit.
There are some great kits out there- austinhomebrew.com for one has hundreds and hundreds of them. You can buy a "clone kit" of just about any commercial beer you could think of, as well as any basic style kit. These ingredient kits are well made with fresh ingredients (no prehopped canned LME junk), fresh grains, fresh yeast, etc, so there is no need to mess around with fixing them up.
If you're not liking the kits you've had in the past, you could always try a good quality kit. Sometimes adding more DME is ok, but usually it'll "unbalance" the flavors in a recipe. Unless you're adding more hops to balance the extra fermentables, just adding DME and/or grain may not be much of an improvement.
