Sure you can, but the cost is way too high. If you can get someone to do it, and that is a BIG if, they would charge more then what new tempered glass would be. I sometimes have my suppliers temper glass that I cut, because it's a crazy pattern or something, and they charge me as much as it would cost if I just bought the glass from them in the first place. The problem is is that the tempering ovens have to be such down and cleaned if the glass breaks in the process. The risk of down time and all the other BS that would go along with it is what you would be paying for and that would be a lot, that is if they would want to deal with it in the first place.
Edit: Tempering glass is the process of heating the glass and then cooling it with air jets so that the surface is cooled faster then the inside. This puts a surface tension on the glass which once broken shatters the entire piece. If you reheated the glass and let it cool uniformly then the surface tension would no longer be there allowing you to cut it. For example, if one has tempered glass in a fire place it will, over time, loose it's tempering.