Adding a fuller, strictly speaking, does not strengthen anything. It was originally a way to not have steel in a place on swords where it didn't matter as much (ie. the centerline of the sword, where it is needed less). Essentially, for a forged sword, more of the steel could go to where it was needed, resulting in a more efficient and strong sword by mass, as well as allowing for a stiffer sword by mass. A fuller was a design element (incorporated into the design of the blade at the start) by which a stronger/stiffer sword could be forged with a set amount of mass, but that's different from grinding a fuller into a finished blade.
The common example of an I-beam "adding strength" is a misunderstanding of what is occurring here. A solid beam with the same outer dimensions as an I-beam is stronger than the I-beam in every respect, particularly along the weak axis of the I-beam. However, the solid beam weighs a lot more and has more in the way of materials and labor costs, so that's why you don't see solid beams in structural design as often.
Grinding a fuller into a finished folding knife blade made by stock removal does not make it stronger. However, it might not make it significantly weaker proportional to how much lighter the fullered blade is, depending on how you do it and in which directions it needs to be strong. The fullers on modern pocket knives are entirely for the sake of aesthetics, or in rare cases to allow for a way of opening the knife.
