You can, but if you are making regular bar soap, there's really no need to do so since the saponification reaction produces glycerin as part of the process. As far as how much glycerin ends up in our finished soaps from saponification, I've read estimates of anywhere between 10% to 15% or so, depending on the fats used in your formula.
Having said that, I add extra glycerin to my shave soaps/croaps (20%) because it helps to sustain the foamy shave lather/make it last longer so that one doesn't have to constantly keep re-applying more soap to last through a shave. It also feels good to freshly-shaved skin.
The one thing I would add to IL's reply is the consequence of having too much glycerin: sweating soap. Actually, handmade bar soaps have some tendency to sweat in humid conditions anyway. You don't want to encourage it.