First, it is possible to correct the body of your beer at bottling time . . . to a certain extent. However, you have to do it correctly.
For instance, corn sugar will definitely not work and DME will probably not get you what you are looking for. You want something that is non-fermentable. I suggest adding some lactose - milk sugar. This is entirely unfermentable and will give you the body and mouthfeel associated with non-fermented sugars.
Be careful though. Remember that carbonation will increase perceived body/mouthfeel. In other words, what seems like a dry beer as you rack to secondary may turn out perfectly after some bottle conditioning. I would leave it alone.
In the future, if you are doing extract batches, you should think about your specialty grains/etc. and what characteristics they add to your beer. If you are all-grain, raise your mash temp a few degrees.